Formof 4.00% Fixed-to-Floating Rate Subordinated Notes due 2031 (included in Exhibit4.2)

Exhibit 4.3

THIS SECURITY IS AN UNSECURED SUBORDINATED DEBT OBLIGATION OF RBB BANCORP (THE “COMPANY”). THIS SECURITY IS NOT A DEPOSIT OR SAVINGS ACCOUNT AND IS NOT INSURED BY THE FEDERAL DEPOSIT INSURANCE CORPORATION OR ANY OTHER GOVERNMENTAL AGENCY OR INSTRUMENTALITY.

THIS SECURITY IS A GLOBAL SECURITY WITHIN THE MEANING OF THE INDENTURE HEREINAFTER REFERRED TO AND IS REGISTERED IN THE NAME OF A DEPOSITARY OR A NOMINEE OF THE DEPOSITARY, WHICH SHALL BE TREATED BY THE COMPANY, THE TRUSTEE AND ANY AGENT THEREOF AS OWNER AND HOLDER OF THIS SECURITY FOR ALL PURPOSES.

UNLESS THIS CERTIFICATE IS PRESENTED BY AN AUTHORIZED REPRESENTATIVE OF THE DEPOSITARY TO THE COMPANY OR ITS AGENT FOR REGISTRATION OF TRANSFER, EXCHANGE OR PAYMENT, AND ANY CERTIFICATE ISSUED IS REGISTERED IN THE NAME OF THE NOMINEE OF THE DEPOSITARY OR IN SUCH OTHER NAME AS IS REQUESTED BY AN AUTHORIZED REPRESENTATIVE OF THE DEPOSITARY (AND ANY PAYMENT HEREON IS MADE TO THE NOMINEE OF THE DEPOSITARY OR TO SUCH OTHER ENTITY AS IS REQUESTED BY AN AUTHORIZED REPRESENTATIVE OF THE DEPOSITARY), ANY TRANSFER, PLEDGE OR OTHER USE HEREOF FOR VALUE OR OTHERWISE BY OR TO ANY PERSON IS WRONGFUL SINCE THE REGISTERED OWNER HEREOF, THE NOMINEE OF THE DEPOSITARY, HAS AN INTEREST HEREIN.

TRANSFERS OF THIS GLOBAL SECURITY SHALL BE LIMITED TO TRANSFERS IN WHOLE, BUT NOT IN PART, BY THE DEPOSITARY TO A NOMINEE OF THE DEPOSITARY, OR BY A NOMINEE OF THE DEPOSITARY TO THE DEPOSITARY OR ANOTHER NOMINEE OF THE DEPOSITARY, OR BY THE DEPOSITARY OR ANY SUCH NOMINEE TO A SUCCESSOR DEPOSITARY OR A NOMINEE OF SUCH SUCCESSOR DEPOSITARY.

RBB BANCORP

4.00% Fixed-to-Floating Rate Subordinated Notes due 2031

CUSIP NO. 74930B A G0
ISIN NO. US74930BAG05

RBB BANCORP, a corporation duly organized and existing under the laws of the State of California (herein called the “Company,” which term includes any successor Person under the Indenture hereinafter referred to), for value received, hereby promises to pay to CEDE & CO., or registered assigns, the principal sum of ONE HUNDRED TWENTY MILLION U.S. Dollars (as may be increased or decreased as reflected on the Schedule of Increases and Decreases in Global Security attached hereto) on April 1, 2031 (such date is hereinafter referred to as the “Stated Maturity Date”), unless redeemed prior to such date, and to pay interest thereon (i) from, and including, March 26, 2021, to, but excluding, April 1, 2026, unless redeemed prior to such date, at a rate of 4.00% per annum, semi-annually in arrears on October 1 and April 1 of each year, commencing October 1, 2021 (each such date, a “Fixed Rate Interest Payment Date,” with the period from, and including, March 26, 2021 to, but excluding, the first Fixed Rate Interest Payment Date and each successive period from, and including, a Fixed Rate Interest Payment Date to, but excluding, the next Fixed Rate Interest Payment Date being a “Fixed Rate Period”) and (ii) from, and including, April 1, 2026 to, but excluding, the Stated Maturity Date, unless redeemed subsequent to April 1, 2026 but prior to the Stated Maturity Date, at a rate equal to Three-Month Term SOFR, reset quarterly, plus 329 basis points, or such other rate as determined pursuant to the Supplemental Indenture, payable quarterly in arrears on April 1, July 1, October 1 and January 1 of each year through the Stated Maturity Date or earlier Redemption Date (each, a “Floating Rate Interest Payment Date” and, together with the Fixed Rate Interest Payment Dates, the “Interest Payment Dates,” with the period from, and including, April 1, 2026 to, but excluding, the first Floating Rate Interest Payment Date and each successive period from, and including a Floating Rate Interest Payment Date to, but excluding, the next Floating Rate Interest Payment Date being a “Floating Rate Period”). The amount of interest payable on any Fixed Rate Interest Payment Date during the Fixed Rate Period will be computed on the basis of a 360-day year consisting of twelve 30-day months up to, but excluding April 1, 2026, and, the amount of interest payable on any Floating Rate Interest Payment Date during the Floating Rate Period will be computed on the basis of a 360-day year and the number of days actually elapsed. In the event that any scheduled Interest Payment Date for this Security falls on a day that is not a Business Day, then payment of interest payable on such Interest Payment Date will be paid on the next succeeding day which is a Business Day (any payment made on such date will be treated as being made on the date that the payment was first due and no interest on such payment will accrue for the period from and after such scheduled Interest Payment Date); provided, that in the event that any scheduled Floating Rate Interest Payment Date falls on a day that is not a Business Day and the next succeeding Business Day falls in the next succeeding calendar month, such Floating Rate Interest Payment Date will be accelerated to the immediately preceding Business Day, and, in each such case, the amounts payable on such Business Day will include interest accrued to, but excluding, such Business Day. All percentages used in or resulting from any calculation of Three-Month Term SOFR shall be rounded, if necessary, to the nearest one hundred-thousandth of a percentage point, with 0.000005% rounded up to 0.00001%.

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 Security (or one or more Predecessor Securities) is registered at the close of business on the fifteenth day (whether or not a Business Day) immediately preceding the applicable 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 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 Company, notice whereof shall be given to Holders of 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 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.

Payment of the principal of (and premium, if any) and interest on this Security will be made at the Corporate Trust Office of the Trustee or at the office of any paying agent that the Company may designate, 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.

Reference is hereby made to the further provisions of this 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 Security shall not be entitled to any benefit under the Indenture or be valid or obligatory for any purpose.

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

[Signature Page Follows]

(Trustee’s Certificate of Authentication)

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

REVERSE SIDE OF THE NOTE

RBB BANCORP

4.00% Fixed-to-Floating Rate Subordinated Notes due 2031

This Security is one of a duly authorized issue of securities of the Company (herein called the “Securities”), issued and to be issued in one or more Series under an Indenture, dated as of March 26, 2021, as supplemented by a First Supplemental Indenture dated as of March 26, 2021 (herein collectively called the “Indenture,” which term shall have the meaning assigned to it in such instrument), between the Company and Wilmington Trust, National Association, as Trustee (herein called the “Trustee”, which term includes any successor trustee under the Indenture), and reference is hereby made to the Indenture (as amended from time to time) for a statement of the respective rights, limitations of rights, duties and immunities thereunder of the Company, the Trustee, the holders of Senior Indebtedness 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.

The indebtedness evidenced by this Security is, to the extent provided in the Indenture, subordinate and subject in right of payment to the prior payment in full of all Senior Indebtedness, and this Security is issued subject to the provisions of the Indenture with respect thereto. Each Holder of this Security, by accepting the same, (a) agrees to and shall be bound by such provisions, (b) authorizes and directs the Trustee on his or her behalf to take such actions as may be necessary or appropriate to effectuate the subordination so provided and (c) appoints the Trustee his or her attorney-in-fact for any and all such purposes. Each Holder hereof, by his or her acceptance hereof, waives all notice of the acceptance of the subordination provisions contained herein and in the Indenture by each holder of Senior Indebtedness, whether now outstanding or hereafter created, incurred, assumed or guaranteed, and waives reliance by each such holder upon said provisions.

The Indenture contains provisions for defeasance at any time 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.

If an Event of Default with respect to Securities of this Series shall occur and be continuing, the principal of the Securities shall be immediately accelerated in the manner and with the effect provided in the Indenture.

The Company may, at its option, redeem the Securities, in whole or in part, at a redemption price equal to 100% of the principal amount of the Securities to be redeemed, plus accrued and unpaid interest (the “Redemption Price”) to, but excluding, the date of redemption (the “Redemption Date”), on any Interest Payment Date on or after April 1, 2026. The Company may also, at its option, redeem the Securities before the Stated Maturity Date, in whole, but not in part, at any time, upon the occurrence of a Tier 2 Capital Event, a Tax Event or if the Company is required to register as an investment company pursuant to the Investment Company Act of 1940, as amended. Any such redemption will be at a redemption price equal to the Redemption Price to, but excluding, the Redemption Date fixed by the Company.

Notwithstanding any of the foregoing, to the extent then required under or pursuant to applicable regulations of the Federal Reserve (or, as and if applicable, the rules of any appropriate successor bank regulatory agency), this Security may not be repaid prior to the Stated Maturity Date without the prior written consent of the Federal Reserve (or, as and if applicable, the rules of any appropriate successor bank regulatory agency). 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 provisions of Article III of the Base Indenture and Section 3.5 of the Supplemental Indenture shall apply to the redemption of any Securities by the Company.

In the event that any payment on the Securities is subject to withholding of any U.S. federal income tax or other tax or assessment (as a result of a change in law or otherwise), the Company will not pay additional amounts with respect to such tax or assessment.

The Indenture permits, with certain exceptions as therein provided, the amendment thereof and the modification of the rights and obligations of the Company and the rights of the Holders of the Securities at any time by the Company and the Trustee with the consent of the Holders of 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 a specified percentage in principal amount of the Securities at the time Outstanding, on behalf of the Holders of all Securities, to waive compliance by the Company 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 of this Series, the Holders of not less than 25% in principal amount of the Securities of this Series at the time Outstanding shall have made written request to the Trustee to institute proceedings in respect of such Event of Default and offered the Trustee indemnity satisfactory to it, and the Trustee shall not have received from the Holders of a majority in principal amount of Securities of this Series at the time 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 Company, 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 Security Register, upon surrender of this Security for registration of transfer at the office or agency of the Company maintained under Section 2.4 of the Indenture for such purpose, duly endorsed by, or accompanied by a written instrument of transfer in form satisfactory to the Company and the Registrar duly executed by, the Holder hereof or his attorney duly authorized in writing, and thereupon one or more new Securities of this Series 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 of this Series are issuable only in registered form without coupons in minimum denominations of $1,000 and integral multiples of $1,000 in excess thereof. As provided in the Indenture and subject to certain limitations therein set forth, Securities of this Series are exchangeable for a like aggregate principal amount of Securities of this Series 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 the Company 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 Company, the Trustee and any agent of the Company 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 Company, the Trustee nor any such agent shall be affected by notice to the contrary.

This Security shall be governed by and construed in accordance with the laws of the State of New York.

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

SCHEDULE OF INCREASES AND DECREASES IN GLOBAL SECURITY

The initial principal amount of this Global Security is $120,000,000. The following increases and decreases in this Global Security have been made: