Nineteenth Supplemental Indenture, dated as of September 23, 2019, between MPLX LP and the Bank of New York Mellon Trust Company, N.A

EXHIBIT 4.3

NINETEENTH SUPPLEMENTAL INDENTURE

THIS NINETEENTH SUPPLEMENTAL INDENTURE, dated as of September 23, 2019 (this “Supplemental Indenture”), is between MPLX LP, a limited partnership duly formed and existing under the laws of the State of Delaware (the “Partnership”), and The Bank of New York Mellon Trust Company, N.A., a national banking association, as trustee (the “Trustee”).

WITNESSETH

WHEREAS, pursuant to the Senior Indenture, dated as of February 12, 2015 (the “Base Indenture”), as supplemented by the First Supplemental Indenture, dated as of February 12, 2015, the Second Supplemental Indenture, dated as of December 22, 2015, the Third Supplemental Indenture, dated as of December 22, 2015, the Fourth Supplemental Indenture, dated as of December 22, 2015, the Fifth Supplemental Indenture, dated as of December 22, 2015, the Sixth Supplemental Indenture, dated as of February 10, 2017, the Seventh Supplemental Indenture, dated as of February 10, 2017, the Eighth Supplemental Indenture, dated as of February 8, 2018, the Ninth Supplemental Indenture, dated as of February 8, 2018, the Tenth Supplemental Indenture, dated as of February 8, 2018, the Eleventh Supplemental Indenture, dated as of February 8, 2018, the Twelfth Supplemental Indenture, dated as of February 8, 2018, the Thirteenth Supplemental Indenture, dated as of November 15, 2018, the Fourteenth Supplemental Indenture, dated as of November 15, 2018, the Fifteenth Supplemental Indenture, dated as of September 9, 2019, the Sixteenth Supplemental Indenture, dated as of September 9, 2019, the Seventeenth Supplemental Indenture, dated as of September 23, 2019, and the Eighteenth Supplemental Indenture, dated as of September 23, 2019 (as so supplemented, the “Indenture”), in each case between the Partnership and the Trustee, the Partnership may from time to time issue and sell Debt Securities in one or more series;

WHEREAS, the Partnership desires to create and authorize a new series of Debt Securities entitled “6.375% Senior Notes due 2024” (the “Notes”), limited initially to $380,521,000 in aggregate principal amount, and to provide the terms and conditions upon which the Notes are to be executed, registered, authenticated, issued and delivered, the Partnership has duly authorized the execution and delivery of this Supplemental Indenture;

WHEREAS, the Notes are a series of Debt Securities and are being issued under the Indenture, as supplemented by this Supplemental Indenture, and are subject to the terms contained therein and herein;

WHEREAS, the Notes are to be substantially in the form attached hereto as Exhibit A; and

WHEREAS, all acts and things necessary to make the Notes, when executed by the Partnership and authenticated and delivered by or on behalf of the Trustee as provided in the Indenture and this Supplemental Indenture, the valid, binding and legal obligations of the Partnership, and to make this Supplemental Indenture a legal, binding and enforceable agreement, have been done and performed.

NOW, THEREFORE, in order to declare the terms and conditions upon which the Notes are executed, registered, authenticated, issued and delivered, and in consideration of the foregoing premises and the purchase of such Notes by the Holders thereof, the Partnership and the Trustee mutually covenant and agree, for the equal and proportionate benefit of the Holders from time to time of the Notes, as follows:

Section 1. Definitions. Terms used in this Supplemental Indenture and not defined herein shall have the respective meanings given such terms in the Indenture.

Section 2. Other Definitions. In addition, the following terms shall have the following meanings with respect to the Notes:

“Additional Interest” means “Additional Interest” payable pursuant to the Registration Rights Agreement.

“Board Resolution” means a copy of a resolution or resolutions of the Board of Directors, certified by the Secretary or an Assistant Secretary of the General Partner to be in full force and effect on the date of such certification, and delivered to the Trustee.

“Clearstream” means Clearstream Banking, société anonyme, or any successor securities clearing agency.

“Consolidated Net Tangible Assets” means the aggregate value of all assets of the Partnership and its Subsidiaries after deducting therefrom: (i) all current liabilities, excluding all short-term indebtedness and the current portion of long-term indebtedness; (ii) all investments in unconsolidated subsidiaries and all investments accounted for on the equity basis; and (iii) all goodwill patents and trademarks, unamortized debt discounts and other similar intangibles (all determined in conformity with generally accepted accounting principles and calculated on a basis consistent with the Partnership’s most recent audited consolidated financial statements).

“Euroclear” means Euroclear Bank S.A./N.V., as operator of Euroclear systems Clearance System or any successor securities clearing agency.

“Exchange Global Notes” has the meaning set forth in Section 7(a) herein.

“Exchange Notes” means any notes issued in exchange for Notes pursuant to the Registration Rights Agreement or similar agreement.

“Global Notes” has the meaning set forth in Section 7(c) herein.

“Interest Payment Date,” when used with respect to any installment of interest on any Debt Security, means the date specified in such Debt Security as the fixed date on which such installment of interest is due and payable.

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“Mortgage” means as the context may require, (i) to mortgage, pledge, encumber or subject to a lien or (ii) a mortgage, pledge, encumbrance or lien.

“Predecessor Security” of any particular Debt Security means every previous Debt Security evidencing all or a portion of the same debt as that evidenced by such particular Debt Security; and, for the purposes of this definition, any Debt Security authenticated and delivered under Section 2.09 of the Base Indenture in exchange for or in lieu of a mutilated, destroyed, lost or stolen Debt Security shall be deemed to evidence the same debt as the mutilated, destroyed, lost or stolen Debt Security.

“QIB” means a “qualified institutional buyer” as defined in Rule 144A.

“Registration Rights Agreement” means the registration rights agreement, dated as of the date of this Supplemental Indenture, by and among the Partnership, the General Partner, Barclays Capital Inc., MUFG Securities Americas Inc. and Wells Fargo Securities, LLC.

“Regular Record Date” for the interest payable on any Interest Payment Date on the Debt Securities means the date specified for that purpose as contemplated by Section 2.03 of the Base Indenture.

“Regulation S” means Regulation S promulgated under the Securities Act.

“Regulation S Global Notes” has the meaning set forth in Section 7(c) herein.

“Regulation S Notes” has the meaning set forth in Section 7(c) herein.

“Restricted Notes Legend” has the meaning set forth in Section 7(d) herein.

“Restricted Period” has the meaning set forth in Section 7(c) herein.

“Rule 144” means Rule 144 promulgated under the Securities Act.

“Rule 144A” means Rule 144A promulgated under the Securities Act.

“Rule 144A Global Notes” has the meaning set forth in Section 7(b) herein.

“Rule 144A Notes” has the meaning set forth in Section 7(b) herein.

“Special Record Date” for the payment of any Defaulted Interest means a date fixed by the Trustee pursuant to Section 2.17 of the Base Indenture.

“Transfer Restricted Note” means any Note that bears or is required to bear the Restricted Notes Legend.

“U.S. person” means a “U.S. person” as defined in Regulation S.

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Section 3. Creation and Authorization of Series.

(a) There is hereby created and authorized the following new series of Debt Securities to be issued under the Indenture, to be designated as the “6.375% Senior Notes due 2024.”

(b) The Notes shall be limited initially to $380,521,000 in aggregate principal amount. Notwithstanding the foregoing initial aggregate principal amount, the Partnership may, from time to time, without notice to or consent of the Holders of the Notes, increase the principal amount of the Notes that may be issued under this Supplemental Indenture and issue such increased principal amount (or any portion thereof), in which case any additional Notes so issued will have the same terms and conditions other than the public offering price, original interest accrual date and initial interest payment date, and the same CUSIP numbers as the applicable series of Notes previously issued, will be fungible with the applicable series of Notes previously issued for U.S. federal income tax purposes, and will carry the same right to receive accrued and unpaid interest as the Notes previously issued, and such additional notes will form a single series with the Notes of such series previously issued, including, without limitation, for purposes of waivers, amendments, redemptions and, if any, offers to purchase, and will rank equally and ratably with the Notes of such series previously issued.

(c) The date on which the principal is payable on the Notes, unless accelerated pursuant to the Indenture, shall be as provided in the form of security attached hereto as Exhibit A.

(d) The Notes shall bear interest as provided in the form of security attached hereto as Exhibit A. The Interest Payment Dates and the Regular Record Dates for the determination of Holders of the Notes to whom such interest is payable shall be as provided in the form of security attached hereto as Exhibit A. Additional Interest, if any, will be payable in arrears on each Interest Payment Date following accrual in the same manner as regular interest on the Notes.

(e) The Notes shall be redeemable at the option of the Partnership as set forth in the form of security attached hereto as Exhibit A.

(f) The provisions of Sections 3.04 and 3.05 of the Base Indenture shall not be applicable to the Notes.

(g) The Notes will be issued only in fully registered form, without coupons, in denominations provided herein and in the form of security attached hereto as Exhibit A.

(h) The Events of Default and covenants specified in the Indenture will apply to the Notes.

(i) The defeasance and covenant defeasance provisions of Article XI of the Indenture, including both Sections 11.02 and 11.03 of the Base Indenture, will apply to the Notes.

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(j) The Notes shall be issued in the form of one or more Global Securities substantially in the form of security attached hereto as Exhibit A. The Partnership initially appoints The Depository Trust Company to act as Depositary with respect to the Notes. Additional provisions applicable to the Notes issued in the form of a Global Security are set forth in the form of security attached hereto as Exhibit A.

(k) The Notes shall be issuable only in denominations of $2,000 and integral multiples of $1,000 in excess thereof.

(l) The Trustee will initially act as the Registrar for the Notes and as the paying agent with respect to the Notes. The Place of Payment will be The Bank of New York Mellon Trust Company, N.A., 240 Greenwich Street, New York, New York 10286.

(m) Except as otherwise set forth herein and in the Notes, the terms of the Notes shall be as set forth in the Indenture, including those made part of the Indenture by reference to the TIA.

Section 4. Additional Covenants. In addition to the covenants set forth in Article IV of the Indenture, the following additional covenants shall apply with respect to the Notes:

Section 4.10 Mortgage of Certain Property.

If the Partnership or any Subsidiary of the Partnership shall Mortgage as security for any indebtedness for money borrowed any pipeline, terminal or other logistics or storage property or asset employed in the transportation, distribution, storage, terminalling, processing or marketing of crude oil, natural gas, condensate or refined products that (i) is located in the United States and (ii) is determined to be a principal property by the Board of Directors in its discretion, the Partnership will secure, or will cause such Subsidiary to secure, the Notes and all other Debt Securities equally and ratably with all indebtedness or obligations secured by the Mortgage then being given; provided, however, that this covenant shall not apply in the case of:

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Notwithstanding the foregoing restriction contained in this Section 4.10, the Partnership may and may permit its Subsidiaries to incur liens or grant Mortgages on property covered by the restriction above so long as the net book value of the property so encumbered, together with all property subject to the restriction on sale and leasebacks contained in Section 4.11, does not, at the time such lien or Mortgage is granted, exceed fifteen percent (15%) of Consolidated Net Tangible Assets.

Section 4.11 Sale and Leaseback of Certain Properties.

The Partnership will not, nor will it permit any Subsidiary of the Partnership to, sell or transfer any pipeline, terminal or other logistics or storage property or asset employed in the transportation, distribution, storage, terminalling, processing or marketing of crude oil, natural gas, condensate or refined products that (i) is located in the United States and (ii) is determined to be a principal property by the Board of Directors in its discretion, with the intention of taking back a lease of such property; provided, however, this covenant shall not apply if:

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In the event of such a sale or transfer, the Partnership shall deliver to the Trustee the Board Resolution referred to in the parenthetical phrase contained in subclause (4)(B) of this Section 4.11 and an Officer’s Certificate setting forth all material facts under this Section 4.11.

For the purposes of this Section 4.11, the term retirement of such funded debt shall include the “in substance defeasance” of such funded debt in accordance with then applicable accounting rules.

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Section 5. Consolidation, Merger, Sale or Conveyance. With respect to the Notes, Section 10.01 of the Base Indenture shall be amended to include the following language at the end of such section:

For purposes of this Section 10.01, “substantially all of its assets” shall mean, at any date, a portion of the non-current assets reflected in the Partnership’s consolidated balance sheet as of the end of the most recent quarterly period that represents at least sixty-six and two-thirds percent (66 2/3%) of the total reported value of such assets.

Section 6. Transfer Restrictions.

(a) The initial offering and sale of the Notes shall not be registered under the Securities Act or any state securities laws. The Notes shall be offered pursuant to exemptions from the registration requirements of the Securities Act in reliance upon Rule 144A and Regulation S promulgated under the Securities Act. For so long as any of the Notes constitute “restricted securities” within the meaning of Rule 144(a)(3) promulgated under the Securities Act, the Partnership shall, if the Partnership is not then subject to the reporting requirements of Section 13 or 15(d) of the Exchange Act, furnish to any Holder or beneficial owner of such Notes, or to any prospective purchaser of such Notes designated by such Holder or beneficial owner, in each case upon the written request of such Holder, beneficial owner or prospective purchaser, the information required to be provided pursuant to Rule 144A(d)(4) promulgated under the Securities Act.

(b) The following provisions shall apply with respect to any proposed transfer of any Transfer Restricted Note prior to the expiration of the holding period applicable to sales of such Notes under Rule 144, and the Registrar shall refuse to register any transfer of such Notes not complying with the restrictions set forth in the Restricted Notes Legend and in this Section 6. In addition to the requirements set forth in Section 2.07 of the Base Indenture, Transfer Restricted Notes that are presented or surrendered for registration of transfer or exchange pursuant to Section 2.07 of the Base Indenture shall be accompanied by the following additional information and documents, as applicable, upon which the Registrar may conclusively rely:

(i) if such Transfer Restricted Notes are being delivered to the Registrar by a Holder for registration in the name of such Holder, without transfer, a certification from such Holder to that effect (in substantially the form of Exhibit B hereto);

(ii) if such Transfer Restricted Notes are being transferred (1) to a QIB in accordance with Rule 144A or (2) pursuant to an effective registration statement under the Securities Act, a certification to that effect from such Holder (in substantially the form of Exhibit B hereto);

(iii) if such Transfer Restricted Notes are being transferred pursuant to an exemption from registration in accordance with Rule 903 or Rule 904 of Regulation S, certifications to that effect from such Holder (in substantially the form of Exhibit B and Exhibit C hereto) and an opinion of counsel to that effect if the Partnership or the Trustee so requests; or

(iv) if such Transfer Restricted Notes are being transferred in reliance on and in compliance with (1) an exemption from registration in accordance with Rule 144 under the Securities Act or (2) another exemption from the registration requirements of the Securities Act, a certification to that effect from such Holder (in substantially the form of Exhibit B attached hereto) and an opinion of counsel, certification or other evidence as may reasonably be required to that effect if the Partnership or the Trustee so requests.

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(c) A Holder of a beneficial interest in a Regulation S Global Note who wishes to transfer its interest in such Note to a QIB in accordance with Rule 144A who takes delivery in the form of a beneficial interest in the Rule 144A Global Note shall deliver to the Registrar a certification to that effect (in substantially the form of Exhibit B attached hereto) upon which the Registrar may conclusively rely. After the expiration of the Restricted Period, interests in the Regulation S Global Note may be transferred without requiring the certification set forth in this Section 6(c).

(d) The transfer and exchange of Global Notes or beneficial interests therein shall be effected through the Depositary, in accordance with Section 2.07 of the Base Indenture and Section 6 and Section 7 hereof (including the restrictions on transfer set forth therein and herein) and the rules and procedures of the Depositary therefor, which shall include restrictions on transfer comparable to those set forth therein and herein to the extent required by the Securities Act; provided, however, that prior to the expiration of the Restricted Period, transfers and exchanges of beneficial interests in the Regulation S Global Note may be made pursuant to such restrictions only (1) to a Person that is not a U.S. person or for the account or benefit of a Person that is not a U.S. person within the meaning of Regulation S under the Securities Act or (2) to a QIB, in each case that hold such interests through Euroclear or Clearstream.

(e) If Notes are issued upon the registration of transfer, exchange or replacement of Notes not bearing the Restricted Notes Legend, the Notes so issued shall not bear such legend. If Notes are issued upon the registration or transfer, exchange or replacement of Notes bearing the Restricted Notes Legend, or if a request is made to remove the Restricted Notes Legend on a Note, the Notes so issued shall bear the Restricted Notes Legend, or the Restricted Notes Legend shall not be removed, as the case may be, unless there is delivered to the Partnership such satisfactory evidence, which may include an opinion of counsel of recognized standing licensed to practice law in the State of New York and experienced in matters involving the Securities Act, as may be reasonably required by the Partnership that neither the Restricted Notes Legend nor the restrictions on transfer set forth therein are required to ensure that transfers thereof comply with the provisions of Rule 144A, Rule 144 or Regulation S, that such Notes are not “restricted securities” within the meaning of Rule 144 or that such Notes were transferred pursuant to an effective registration statement under the Securities Act. Upon provision of such satisfactory evidence, the Trustee, at the direction of the Partnership, shall authenticate and deliver a Note that does not bear the Restricted Notes Legend. If a Restricted Notes Legend is removed from the face of a Note and the Note is subsequently held by an Affiliate of the Partnership, the Restricted Notes Legend shall be reinstated.

(f) Neither the Trustee nor the Registrar shall have any obligation or duty to monitor, determine or inquire as to compliance with any restrictions on transfer imposed under this Supplemental Indenture or under applicable law with respect to any transfer of any interest in any Note (including any transfers between or among Depositary participants or beneficial owners of interests in any Global Note) other than to require delivery of such certificates and other documentation or evidence as are expressly required by, and to do so only if and when expressly required by the terms of, this Supplemental Indenture, and to examine the same to determine

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substantial compliance as to form with the express requirements hereof. Notwithstanding anything herein to the contrary, neither the Trustee nor the Registrar shall have any responsibility to receive any letters, opinions or certifications, nor any responsibility to monitor compliance with any transfer restrictions, in connection with any transfer or exchange of any beneficial interest in a Global Note for a beneficial interest in the same Global Note.

(g) Notwithstanding the foregoing, in the event that any Transfer Restricted Notes are exchanged for Exchange Notes in connection with an effective registration statement pursuant to the Registration Rights Agreement, the Partnership shall issue and, at the direction of the Partnership, the Trustee shall authenticate the Exchange Notes in exchange for Transfer Restricted Notes accepted for exchange in the exchange offer, which Exchange Notes shall not bear the Restricted Notes Legend, and the Registrar shall rescind any restriction on the transfer of such Exchange Notes.

Section 7. Form of Notes/Legends.

(a) The Notes are being offered only to (A) Persons reasonably believed to be QIBs in reliance on Rule 144A and (B) non-U.S. persons in reliance on Regulation S. Such Notes may thereafter be transferred to, among others, QIBs, and purchasers in reliance on Regulation S, in each case, in accordance with the procedures described herein. Exchange Notes exchanged for interests in the Rule 144A Notes and the Regulation S Notes will be issued in the form of permanent global notes deposited with the Trustee as hereinafter provided, including the appropriate legend set forth in the form of security attached as Exhibit A (the “Exchange Global Notes”). The Exchange Global Notes will be deposited upon issuance with, or on behalf of, the Trustee as custodian for the Depositary, duly executed by the Partnership and authenticated by the Trustee as hereinafter provided. The Exchange Global Notes may be represented by more than one certificate, if so required by the Depositary’s rules regarding the maximum principal amount to be represented by a single certificate.

(b) The Notes offered to QIBs in the United States of America in reliance on Rule 144A (the “Rule 144A Notes”) shall be issued in the form of a permanent global note including appropriate legends as set forth in the form of security attached as Exhibit A (the “Rule 144A Global Notes”), deposited with the Trustee, as custodian for the Depositary, duly executed by the Partnership and authenticated by the Trustee as hereinafter provided. The Rule 144A Global Notes may be represented by more than one certificate, if so required by the Depositary’s rules regarding the maximum principal amount to be represented by a single certificate. The aggregate principal amount of the Rule 144A Global Notes may from time to time be increased or decreased by adjustments made on the records of the Trustee, as custodian for the Depositary, or its nominee, as hereinafter provided.

(c) The Notes offered to non-U.S. persons outside the United States of America (the “Regulation S Notes”) in reliance on Regulation S shall be issued in the form of a permanent global note including appropriate legends as set forth in the form of security attached as Exhibit A (the “Regulation S Global Notes” and, together with the Rule 144A Global Notes and the Exchange Global Notes, the “Global Notes”). Each Regulation S Global Note will be deposited upon issuance with, or on behalf of, the Trustee as custodian for the Depositary. Prior

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to the 40th day after the date the Notes are issued (such period through and including such 40th day, the “Restricted Period”), interests in the Regulation S Global Notes may only be transferred to non-U.S. persons pursuant to Regulation S, unless exchanged for interests in a Global Note in accordance with the transfer and certification requirements described herein. Investors may hold their interests in the Regulation S Global Note through organizations other than Euroclear or Clearstream that are participants in the Depositary’s system or directly through Euroclear or Clearstream, if they are participants in such systems, or indirectly through organizations which are participants in such systems. If such interests are held through Euroclear or Clearstream, Euroclear and Clearstream will hold such interests in the applicable Regulation S Global Note on behalf of their participants through customers’ securities accounts in their respective names on the books of their respective depositaries. Such depositaries, in turn, will hold such interests in the applicable Regulation S Global Note in customers’ securities accounts in the depositaries’ names on the books of the Depositary. The Regulation S Global Notes may be represented by more than one certificate, if so required by the Depositary’s rules regarding the maximum principal amount to be represented by a single certificate. The aggregate principal amount of the Regulation S Global Notes may from time to time be increased or decreased by adjustments made on the records of the Trustee, as custodian for the Depositary or its nominee, as hereinafter provided.

(d) Unless and until (i) a Note issued as a Transfer Restricted Note is sold under an effective registration statement, (ii) a Note issued as a Transfer Restricted Note is exchanged for an Exchange Note in connection with an effective registration statement pursuant to the Registration Rights Agreement or (iii) the Partnership receives an opinion of counsel satisfactory to it to the effect that neither such legend nor the related restrictions on transfer are required in order to maintain compliance with the provisions of the Securities Act:

(1) each Transfer Restricted Note shall bear the following legend on the face thereof (the “Restricted Notes Legend”):

THIS NOTE HAS NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED (THE “SECURITIES ACT”), AND MAY NOT BE OFFERED, SOLD, PLEDGED OR OTHERWISE TRANSFERRED EXCEPT IN ACCORDANCE WITH THE FOLLOWING SENTENCE. BY ITS ACQUISITION HEREOF OR OF A BENEFICIAL INTEREST HEREIN, THE ACQUIRER (1) REPRESENTS THAT (A) IT AND ANY ACCOUNT FOR WHICH IT IS ACTING IS A “QUALIFIED INSTITUTIONAL BUYER” (WITHIN THE MEANING OF RULE 144A UNDER THE SECURITIES ACT) AND THAT IT EXERCISES SOLE INVESTMENT DISCRETION WITH RESPECT TO EACH SUCH ACCOUNT, OR (B) IT IS NOT A “U.S. PERSON” (WITHIN THE MEANING OF REGULATION S UNDER THE SECURITIES ACT) AND (2) AGREES FOR THE BENEFIT OF MPLX LP THAT IT WILL NOT OFFER, SELL, PLEDGE OR OTHERWISE TRANSFER THIS NOTE OR ANY BENEFICIAL INTEREST HEREIN, EXCEPT IN ACCORDANCE WITH THE SECURITIES ACT AND ANY APPLICABLE SECURITIES LAWS OF ANY STATE OF THE UNITED STATES AND ONLY (A) TO MPLX LP, (B) PURSUANT TO A REGISTRATION STATEMENT WHICH HAS BECOME EFFECTIVE UNDER THE SECURITIES ACT, (C) TO A QUALIFIED INSTITUTIONAL BUYER IN COMPLIANCE WITH RULE 144A UNDER THE SECURITIES ACT, (D) IN AN OFFSHORE TRANSACTION IN

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COMPLIANCE WITH RULE 904 OF REGULATION S UNDER THE SECURITIES ACT, OR (E) PURSUANT TO AN EXEMPTION FROM REGISTRATION PROVIDED BY RULE 144 UNDER THE SECURITIES ACT OR ANY OTHER AVAILABLE EXEMPTION FROM THE REGISTRATION REQUIREMENTS OF THE SECURITIES ACT.

PRIOR TO THE REGISTRATION OF ANY TRANSFER IN ACCORDANCE WITH (2)(C) ABOVE OR (2)(D) ABOVE, A DULY COMPLETED AND SIGNED CERTIFICATE (THE FORM OF WHICH MAY BE OBTAINED FROM THE TRUSTEE) MUST BE DELIVERED TO THE TRUSTEE. PRIOR TO THE REGISTRATION OF ANY TRANSFER IN ACCORDANCE WITH (E) ABOVE, MPLX LP RESERVES THE RIGHT TO REQUIRE THE DELIVERY OF SUCH LEGAL OPINIONS, CERTIFICATIONS OR OTHER EVIDENCE AS MAY REASONABLY BE REQUIRED IN ORDER TO DETERMINE THAT THE PROPOSED TRANSFER IS BEING MADE IN COMPLIANCE WITH THE SECURITIES ACT AND APPLICABLE STATE SECURITIES LAWS. NO REPRESENTATION IS MADE AS TO THE AVAILABILITY OF ANY RULE 144 EXEMPTION FROM THE REGISTRATION REQUIREMENTS OF THE SECURITIES ACT.

(2) each Global Note shall bear the following legend on the face thereof:

UNLESS THIS CERTIFICATE IS PRESENTED BY AN AUTHORIZED REPRESENTATIVE OF THE DEPOSITORY TRUST COMPANY, A NEW YORK CORPORATION (“DTC”), NEW YORK, NEW YORK, TO THE PARTNERSHIP OR ITS AGENT FOR REGISTRATION OF TRANSFER, EXCHANGE OR PAYMENT, AND ANY CERTIFICATE ISSUED IS REGISTERED IN THE NAME OF CEDE & CO. OR SUCH OTHER NAME AS IS REQUESTED BY AN AUTHORIZED REPRESENTATIVE OF DTC (AND ANY PAYMENT IS MADE TO CEDE & CO., OR TO SUCH OTHER ENTITY AS IS REQUESTED BY AN AUTHORIZED REPRESENTATIVE OF DTC), ANY TRANSFER, PLEDGE OR OTHER USE HEREOF FOR VALUE OR OTHERWISE BY OR TO ANY PERSON IS WRONGFUL INASMUCH AS THE REGISTERED OWNER HEREOF, CEDE & CO., HAS AN INTEREST HEREIN.

TRANSFERS OF THIS GLOBAL SECURITY SHALL BE LIMITED TO TRANSFERS IN WHOLE, BUT NOT IN PART, TO NOMINEES OF DTC OR TO A SUCCESSOR THEREOF OR SUCH SUCCESSOR’S NOMINEE AND TRANSFERS OF PORTIONS OF THIS GLOBAL SECURITY SHALL BE LIMITED TO TRANSFERS MADE IN ACCORDANCE WITH THE RESTRICTIONS SET FORTH IN THE INDENTURE REFERRED TO HEREIN.

Section 8. Effect of Supplemental Indenture. The provisions of this Supplemental Indenture are intended to supplement those of the Indenture as in effect immediately prior to the execution and delivery hereof. The Indenture shall remain in full force and effect except to the extent that the provisions of the Indenture are expressly modified by the terms of this Supplemental Indenture.

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Section 9. Governing Law. This Supplemental Indenture and the Notes shall be governed by and construed in accordance with the law of the State of New York, without giving effect to any principles of conflicts of laws thereunder to the extent the application of the laws of another jurisdiction would be required thereby.

Section 10. Trustee Not Responsible for Recitals or Issuance of Notes. The recitals and statements contained herein shall be taken as statements of the Partnership, and the Trustee assumes no responsibility for their correctness. The Trustee makes no representations as to the validity or sufficiency of this Supplemental Indenture or of the Notes other than with respect to the Trustee’s execution of this Supplemental Indenture and authentication of the Notes. The Trustee shall not be accountable for the use or application by the Partnership of the Notes or the proceeds thereof.

Section 11. Conflict with the TIA. If any provision hereof limits, qualifies or conflicts with a provision of the TIA that is required under the TIA to be a part of and govern this Supplemental Indenture, the latter provision shall control. If any provision of this Supplemental Indenture modifies or excludes any provision of the TIA that may be so modified or excluded, the latter provision shall be deemed to apply to this Supplemental Indenture as so modified or to be excluded, as the case may be.

Section 12. Counterparts. This Supplemental Indenture may be executed in any number of counterparts, each of which shall be an original; but such counterparts shall together constitute one and the same instrument. The exchange of copies of this Supplemental Indenture and of signature pages by facsimile or electronic (i.e., “pdf” or “tif”) transmission shall constitute effective execution and delivery of this Supplemental Indenture for all purposes. Signatures of the parties hereto transmitted by facsimile or electronic (i.e., “pdf” or “tif”) transmission shall be deemed to be their original signatures for all purposes of the Supplemental Indenture as to the parties hereto and may be used in lieu of the original.

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IN WITNESS WHEREOF, the parties hereto have caused this Supplemental Indenture to be duly executed as of the day and year first above written.

[Signature Page to Nineteenth Supplemental Indenture]

[Signature Page to Nineteenth Supplemental Indenture]

Exhibit A

MPLX LP

6.375% Senior Notes due 2024

[FOR GLOBAL NOTES: UNLESS THIS CERTIFICATE IS PRESENTED BY AN AUTHORIZED REPRESENTATIVE OF THE DEPOSITORY TRUST COMPANY, A NEW YORK CORPORATION (“DTC”), NEW YORK, NEW YORK, TO THE PARTNERSHIP OR ITS AGENT FOR REGISTRATION OF TRANSFER, EXCHANGE OR PAYMENT, AND ANY CERTIFICATE ISSUED IS REGISTERED IN THE NAME OF CEDE & CO. OR SUCH OTHER NAME AS IS REQUESTED BY AN AUTHORIZED REPRESENTATIVE OF DTC (AND ANY PAYMENT IS MADE TO CEDE & CO., OR TO SUCH OTHER ENTITY AS IS REQUESTED BY AN AUTHORIZED REPRESENTATIVE OF DTC), ANY TRANSFER, PLEDGE OR OTHER USE HEREOF FOR VALUE OR OTHERWISE BY OR TO ANY PERSON IS WRONGFUL INASMUCH AS THE REGISTERED OWNER HEREOF, CEDE & CO., HAS AN INTEREST HEREIN.

TRANSFERS OF THIS GLOBAL SECURITY SHALL BE LIMITED TO TRANSFERS IN WHOLE, BUT NOT IN PART, TO NOMINEES OF DTC OR TO A SUCCESSOR THEREOF OR SUCH SUCCESSOR’S NOMINEE AND TRANSFERS OF PORTIONS OF THIS GLOBAL SECURITY SHALL BE LIMITED TO TRANSFERS MADE IN ACCORDANCE WITH THE RESTRICTIONS SET FORTH IN THE INDENTURE REFERRED TO HEREIN.]

[FOR TRANSFER RESTRICTED NOTES: THIS NOTE HAS NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED (THE “SECURITIES ACT”), AND MAY NOT BE OFFERED, SOLD, PLEDGED OR OTHERWISE TRANSFERRED EXCEPT IN ACCORDANCE WITH THE FOLLOWING SENTENCE. BY ITS ACQUISITION HEREOF OR OF A BENEFICIAL INTEREST HEREIN, THE ACQUIRER (1) REPRESENTS THAT (A) IT AND ANY ACCOUNT FOR WHICH IT IS ACTING IS A “QUALIFIED INSTITUTIONAL BUYER” (WITHIN THE MEANING OF RULE 144A UNDER THE SECURITIES ACT) AND THAT IT EXERCISES SOLE INVESTMENT DISCRETION WITH RESPECT TO EACH SUCH ACCOUNT, OR (B) IT IS NOT A “U.S. PERSON” (WITHIN THE MEANING OF REGULATION S UNDER THE SECURITIES ACT) AND (2) AGREES FOR THE BENEFIT OF MPLX LP THAT IT WILL NOT OFFER, SELL, PLEDGE OR OTHERWISE TRANSFER THIS NOTE OR ANY BENEFICIAL INTEREST HEREIN, EXCEPT IN ACCORDANCE WITH THE SECURITIES ACT

AND ANY APPLICABLE SECURITIES LAWS OF ANY STATE OF THE UNITED STATES AND ONLY (A) TO MPLX LP, (B) PURSUANT TO A REGISTRATION STATEMENT WHICH HAS BECOME EFFECTIVE UNDER THE SECURITIES ACT, (C) TO A QUALIFIED INSTITUTIONAL BUYER IN COMPLIANCE WITH RULE 144A UNDER THE SECURITIES ACT, (D) IN AN OFFSHORE TRANSACTION IN COMPLIANCE WITH RULE 904 OF REGULATION S UNDER THE SECURITIES ACT, OR (E) PURSUANT TO AN EXEMPTION FROM REGISTRATION PROVIDED BY RULE 144 UNDER THE SECURITIES ACT OR ANY OTHER AVAILABLE EXEMPTION FROM THE REGISTRATION REQUIREMENTS OF THE SECURITIES ACT.

PRIOR TO THE REGISTRATION OF ANY TRANSFER IN ACCORDANCE WITH (2)(C) ABOVE OR (2)(D) ABOVE, A DULY COMPLETED AND SIGNED CERTIFICATE (THE FORM OF WHICH MAY BE OBTAINED FROM THE TRUSTEE) MUST BE DELIVERED TO THE TRUSTEE. PRIOR TO THE REGISTRATION OF ANY TRANSFER IN ACCORDANCE WITH (E) ABOVE, MPLX LP RESERVES THE RIGHT TO REQUIRE THE DELIVERY OF SUCH LEGAL OPINIONS, CERTIFICATIONS OR OTHER EVIDENCE AS MAY REASONABLY BE REQUIRED IN ORDER TO DETERMINE THAT THE PROPOSED TRANSFER IS BEING MADE IN COMPLIANCE WITH THE SECURITIES ACT AND APPLICABLE STATE SECURITIES LAWS. NO REPRESENTATION IS MADE AS TO THE AVAILABILITY OF ANY RULE 144 EXEMPTION FROM THE REGISTRATION REQUIREMENTS OF THE SECURITIES ACT.]

MPLX LP, a limited partnership duly formed and existing under the laws of the State of Delaware (herein called the “Partnership,” which term includes any successor Person under the Indenture hereinafter referred to), for value received, hereby promises to pay to [•] [Insert if Global Security: CEDE & CO.], or registered assigns, the principal sum of [•] Dollars ($[•]) [Insert if Global Security: , or such greater or lesser amount as indicated on the Schedule of Increases or Decreases in the Principal Amount of Securities attached hereto,] on May 1, 2024, and to pay interest thereon from May 1, 2019 or from the most recent Interest Payment Date to which interest has been paid or duly provided for, semi-annually on May 1 and November 1 in each year commencing November 1, 2019, at the rate of 6.375% per annum, until the principal hereof is paid or made available for payment. Interest on the Notes will be paid on the basis of a 360-day year consisting of twelve 30-day months. The Partnership shall pay Additional Interest, if any, on the applicable Interest Payment Date in the same manner as interest is paid on the Notes and in the amounts set forth in the Registration Rights Agreement. The interest so payable, and punctually paid or duly provided for, on any Interest Payment Date will, as provided in such Indenture, be paid to the Person in whose name this Debt Security (or one or more Predecessor Securities) is registered at the close of business on the Regular Record Date for such interest, which shall be April 15 or October 15 (whether or not a Business Day), as the case may be, next preceding such Interest Payment Date. Any such interest not so punctually paid or duly provided for will forthwith cease to be payable to the Holder on such Regular Record Date and may either be paid to the Person in whose

name this Debt Security (or one or more Predecessor Securities) is registered at the close of business on a Special Record Date for the payment of such Defaulted Interest to be fixed by the Trustee, notice whereof shall be given to Holders of Debt Securities of this series not less than 10 days prior to such Special Record Date, or be paid at any time in any other lawful manner not inconsistent with the requirements of any securities exchange on which the Debt Securities of this series may be listed, and upon such notice as may be required by such exchange all as more fully provided in said Indenture. If an Interest Payment Date, a Stated Maturity or a Redemption Date with respect to this Debt Security falls on a day that is not a Business Day, the payment will be made on the next Business Day and no interest will accrue for the period from and after such Interest Payment Date, Stated Maturity or Redemption Date.

Payment of the principal of (and premium, if any) and interest on this Debt Security will be made at the office or agency of the Partnership maintained for that purpose in New York, New York, in such coin or currency of the United States of America as at the time of payment is legal tender for payment of public and private debts; provided, however, that (1) payments on any Global Security shall be made by electronic (same-day) funds transfer to the Depositary and (2) at the option of the Partnership payment of interest may be made by check mailed to the address of the Person entitled thereto as such address shall appear in the Debt Security Register or by electronic funds transfer to an account maintained by the Person entitled thereto as specified in the Debt Security Register, provided that such Person shall have given the Trustee written instructions.

Reference is hereby made to the further provisions of this Debt Security set forth on the reverse hereof, which further provisions shall for all purposes have the same effect as if set forth at this place.

Unless the certificate of authentication hereon has been executed by the Trustee referred to on the reverse hereof by manual signature, this Debt Security shall not be entitled to any benefit under the Indenture or be valid or obligatory for any purpose.

IN WITNESS WHEREOF, the Partnership has caused this instrument to be duly executed.

Dated:

CERTIFICATE OF AUTHENTICATION

This is one of the Debt Securities of the series designated therein referred to in the within-mentioned Indenture.

Dated:

MPLX LP

6.375% Senior Notes due 2024

This Security is one of a duly authorized issue of Debt Securities of the Partnership (herein called the “Securities”), issued and to be issued in one or more series under a Senior Indenture, dated as of February 12, 2015, as amended or supplemented to date, including as amended and supplemented by the Nineteenth Supplemental Indenture, dated as of September 23, 2019 (herein called the “Indenture,” which term shall have the meaning assigned to it in such instrument), between the Partnership and The Bank of New York Mellon Trust Company, N.A., as Trustee (herein called the “Trustee,” which term includes any successor trustee under the Indenture), and reference is hereby made to the Indenture for a statement of the respective rights, limitations of rights, duties and immunities thereunder of the Partnership, the Trustee and the Holders of the Securities and of the terms upon which the Securities are, and are to be, authenticated and delivered. This Security is one of the series designated on the face hereof initially limited in aggregate principal amount to $[•].

The Securities are subject to redemption, in whole at any time or in part from time to time, at the election of the Partnership, prior to their maturity date, at the redemption prices (expressed as percentages of the principal amount of such Securities to be redeemed) set forth below, plus accrued and unpaid interest on the principal amount being redeemed to, but not including, the Redemption Date, subject to the right of Holders of record on the relevant record date to receive interest due on the relevant Interest Payment Date, if redeemed during the twelve-month period beginning on May 1 of each of the years indicated below:

Notice of the redemption will be transmitted to holders of Securities at least 15 and not more than 60 days prior to the Redemption Date. If fewer than all of the Securities are to be redeemed, the Trustee will select the particular Securities or portions thereof for redemption from the outstanding Securities not previously called by any of the following methods: (i) in compliance with the requirements of the principal national securities exchange, if any, on which such Securities are listed, (2) on a pro rata basis to the extent practicable and in accordance with the procedures of the Depositary or (3) by lot or in such other similar method in accordance with the procedures of the depositary of such Securities.

Unless the Partnership defaults in payment of the redemption price, on or after the Redemption Date, interest will cease to accrue on the Securities or portions thereof called for redemption.

In the event of redemption of this Security in part only, a new Security or Securities of this series and of like tenor for the unredeemed portion hereof will be issued in the name of the Holder hereof upon the cancellation hereof.

The Indenture contains provisions for defeasance at any time of the entire indebtedness of this Security or of certain restrictive covenants and Events of Default with respect to this Security, in each case upon compliance with certain conditions set forth in the Indenture.

The Indenture contains provisions permitting the Partnership and the Trustee to modify the Indenture or any supplemental indenture without the consent of the Holders for one or more of the following purposes: (1) to evidence the succession of another Person to the Partnership; (2) to surrender any right or power conferred by the Indenture, to add to the covenants for the protection of the Holders of the Securities or to add additional defaults or events of default; (3) to cure any ambiguity, to correct or supplement any provision of the Indenture which may be defective or inconsistent with any other provision of the Indenture, or to make any other provisions with respect to matters or questions arising under the Indenture as shall not adversely affect the interests of the Holders; (4) to permit qualification of the Indenture or any supplemental indenture under the TIA; (5) to permit or facilitate the issuance of Debt Securities of any series in uncertificated form; (6) to secure any or all Debt Securities; (7) to make any change that does not adversely affect the rights of any Holder; (8) to add to, change or eliminate any of the provisions of the Indenture in respect of one or more Debt Securities, under certain conditions specified therein; (9) to evidence and provide acceptance of appointment thereunder by a successor or separate Trustee and to add to or change the provisions under the Indenture as necessary; and (10) to establish the form or terms of Debt Securities of a particular series as permitted by Sections 2.01 and 2.03 of the Indenture.

The Indenture also permits, with certain exceptions as therein provided, the amendment thereof and the modification of the rights and obligations of the Partnership and the rights of the Holders of the Securities of each series to be affected under the Indenture at any time by the Partnership and the Trustee with the consent of the Holders of not less than a majority in aggregate principal amount of the Securities at the time Outstanding of each series to be affected. The Indenture also contains provisions permitting the Holders of at least a majority in aggregate principal amount of the Securities at the time Outstanding, on behalf of the Holders of all Securities, to waive compliance by the Partnership with certain provisions of the Indenture and certain past defaults under the Indenture and their consequences. Any such consent or waiver by the Holder of this Security shall be conclusive and binding upon such Holder and upon all future Holders of this Security and of any Security issued upon the registration of transfer hereof or in exchange herefor or in lieu hereof, whether or not notation of such consent or waiver is made upon this Security.

As provided in and subject to the provisions of the Indenture, the Holder of this Security shall not have the right to institute any proceeding with respect to the Indenture or for the appointment of a receiver or trustee or for any other remedy thereunder, unless such Holder shall have previously given the Trustee written notice of a continuing Event of Default with respect to the Securities, the Holders of not less than 25% in aggregate principal amount of the Securities then Outstanding shall have made written request to the Trustee to institute proceedings in respect of such Event of Default as Trustee and offered the Trustee indemnity, and the Trustee shall not have received from the Holders of a majority in principal amount of the Securities then Outstanding a direction inconsistent with such request, and shall have failed to institute any such proceeding, for 60 days after receipt of such notice, request and offer of indemnity. The foregoing shall not apply to any suit instituted by the Holder of this Security for the enforcement of any payment of principal hereof or any premium or interest hereon on or after the respective due dates expressed herein.

No reference herein to the Indenture and no provision of this Security or of the Indenture shall alter or impair the obligation of the Partnership, which is absolute and unconditional, to pay the principal of and any premium and interest on this Security at the times, place and rate, and in the coin or currency, herein prescribed.

As provided in the Indenture and subject to certain limitations therein set forth, the transfer of this Security is registrable in the Debt Security Register, upon surrender of this Security for registration of transfer at the office or agency of the Partnership in any place where the principal of and any premium and interest on this Security are payable, duly endorsed by, or accompanied by a written instrument of transfer in form satisfactory to the Partnership and the Registrar duly executed by, the Holder hereof or his attorney duly authorized in writing, and thereupon one or more new Securities and of like tenor, of authorized denominations and for the same aggregate principal amount, will be issued to the designated transferee or transferees.

The Securities are issuable only in registered form without coupons in denominations of $2,000 and integral multiples of $1,000 in excess thereof. As provided in the Indenture and subject to certain limitations therein set forth, the Securities are exchangeable for a like aggregate principal amount of Securities and of like tenor of a different authorized denomination, as requested by the Holder surrendering the same.

No service charge shall be made for any such registration of transfer or exchange, but, subject to any applicable provisions of the Indenture, the Partnership may require payment of a sum sufficient to cover any tax or other governmental charge payable in connection therewith.

Prior to due presentment of this Security for registration of transfer, the Partnership, the Trustee and any agent of the Partnership or the Trustee may treat the Person in whose name this Security is registered as the owner hereof for all purposes, whether or not this Security be overdue, and neither the Partnership, the Trustee nor any such agent shall be affected by notice to the contrary.

All terms used in this Security which are defined in the Indenture shall have the meaning assigned to them in the Indenture.

FORM OF ASSIGNMENT

ABBREVIATIONS

Customary abbreviations may be used in the name of a Securityholder or an assignee, such as TEN COM (=tenants in common), TEN ENT (=tenants by the entireties), JT TEN (=joint tenants with rights of survivorship and not as tenants in common), CUST (=custodian), and U/G/M/A (=Uniform Gift to Minors Act).

Additional abbreviations may also be used though not in the above list.