Exhibit 10.18.4

AMENDMENT TO

RISK CAPITAL REINSURANCE COMPANY

EXECUTIVE SUPPLEMENTAL NON-QUALIFIED

SAVINGS AND RETIREMENT PLAN

The Adoption Agreement for the Risk Capital Reinsurance Company Executive Supplemental Non-Qualified Savings and Retirement Plan (the “Plan”) is amended by adding the following new Section 2 to end thereof, effective as of November 23, 1999.

2.        Amendment of Basic Plan Document.

2.01.     Section 8.01(a) of the Basic Plan Document is amended by deleting “deferral election” therefrom and substituting in its place “applicable distribution election under Section 8.02 hereof.”

2.02.     Section 8.01(b) of the Basic Plan Document is amended by deleting the second sentence thereof.

2.03.     Section 8.02 of the Basic Plan Document is amended to read in its entirety as follows:

“8.02    Determination of Method of Distribution.  Notwithstanding any provision of this Plan to the contrary, all benefits under the Plan shall be distributed in a single lump sum in cash as soon as practicable following the Participant’s retirement, death or other termination of employment; provided, however, that a Participant may make an election on or after November 23, 1999 to change the method of distribution for all of such Participant’s benefits under the Plan (whether payable to the Participant or his or her Beneficiary) to any method permitted under Section 8.01; provided further, however, that an election to change the method of distribution under this Section 8.02 shall be effective only if the Participant’s employment terminates more than six months after, and in the calendar year that begins after, the date the written election is filed with the Company by the Participant.”

2.04.     Section 8.04 of the Basic Plan Document is deleted.

2

AMENDMENT TO

ARCH CAPITAL GROUP (U.S.) INC.

EXECUTIVE SUPPLEMENTAL NON-QUALIFIED

SAVINGS AND RETIREMENT PLAN

The Adoption Agreement for the Arch Capital Group (U.S.) Inc. Executive Supplemental Non-Qualified Savings and Retirement Plan, as amended, is hereby amended as set forth below.  Unless otherwise indicated, all capitalized terms used herein shall have the meanings set forth in the Adoption Agreement and the Basic Plan Document.

1.        Rider 3 of the Adoption Agreement.

Rider 3 of the Adoption Agreement is hereby amended to read in its entirety as follows:

Effective from and after the date set forth below, the Employer shall make an additional Deferral Contribution on behalf of each Participant who has entered into a salary reduction agreement with the Employer to defer an amount in addition to the amount set forth in the first sentence of Section 1.05(a) of the Adoption Agreement from his or her Excess Compensation for the Plan Year and/or his or her annual incentive bonus for a Plan Year (whether paid in the Plan Year or a subsequent year).  All additional Deferral Contributions set forth in this paragraph shall not be eligible for Matching Contributions under Section 1.05(b) hereof.

2.        Section 4.01 of the Basic Plan Document

Section 4.01 of the Basic Plan Document is hereby amended by adding the following after the sixth sentence thereof:

Each Participant may elect to execute a salary reduction agreement with the Employer to reduce his or her annual incentive bonus for a Plan Year up to 100% of the annual

incentive bonus.  Such an election with respect to annual incentive bonus (i) must be made no later than the end of the Plan Year immediately preceding the Plan Year in which the bonus is paid; and (ii) shall be irrevocable and shall continue in effect for all future Plan Years until changed in writing, which change must be made prior to the last day for making a deferral election with respect to annual incentive bonus for the Plan Year for which the change will be effective.

AMENDMENT TO THE

ARCH CAPITAL GROUP (U.S.) INC. EXECUTIVE

NON-QUALIFIED SAVINGS AND RETIREMENT PLAN

BASIC PLAN DOCUMENT

The Arch Capital Group (U.S.) Inc. Executive Supplemental Non-Qualified Savings and Retirement Plan Basic Plan Document is hereby amended, effective as of March 1, 2003, as set forth below.

1.        Section 4.01

The seventh sentence of Section 4.01 of the Basic Plan Document is hereby amended and restated as follows:

“Each Participant may elect to execute a salary reduction agreement with the Employer to reduce his or her annual incentive bonus for a Plan Year up to 100% of the annual incentive bonus.  Such an election with respect to annual incentive bonus (i) must be made no later than October 31 of the Plan Year for which the bonus is paid; and (ii) shall be irrevocable and shall continue in effect for all future Plan Years until changed in writing, which change must be made prior to the last day for making a deferral election with respect to annual incentive bonus for the Plan Year for which the change will be effective.”

2.        Section 7.08

New Section 7.08 of the Basic Plan Document is hereby added immediately following Section 7.07 to read as follows:

“7.08 Obligor.  Notwithstanding any provision of this Plan to the contrary, benefits payable under this Plan to a Participant or his or her Beneficiary shall be the obligation of the Employer who actually employs (or, in the case a Participant who is no longer employed by an Employer, last employed) the Participant; provided, however, that in the event the Participant’s employer fails to make a payment of benefits to the Participant or his or her Beneficiary when due under the terms of this Plan, Arch Capital Group Ltd. (the parent company of the Employers) shall be obligated to make such benefit payments in accordance with the terms of this Plan.”

IN WITNESS WHEREOF, Arch Capital Group (U.S.) Inc. has caused this Amendment to be executed this       day of July, 2003.

AMENDMENT TO TRUST AGREEMENT

AMENDMENT TO TRUST AGREEMENT, dated as of the       day of July, 2003, between Arch Capital Group (U.S.) Inc., a Delaware corporation, having an office at 20 Horseneck Lane, Greenwich, Connecticut  06830 (the “Sponsor”) and FIDELITY MANAGEMENT TRUST COMPANY, a Massachusetts trust company, having an office at 82 Devonshire Street, Boston, Massachusetts 02109 (the “Trustee”).

WHEREAS, Arch Reinsurance Company, formerly Risk Capital Reinsurance Company, and the Trustee entered into a Trust Agreement (the “Trust Agreement”) dated as of December 18, 1995 and amended as of February 16, 1996 with respect to the Arch Reinsurance Company Executive Supplemental Non-Qualified Savings and Retirement Plan (the “Arch Re Plan”); and

WHEREAS, effective January 1, 2001, the rights and obligations of Arch Reinsurance Company under the Trust Agreement were assigned to and assumed by the Sponsor; and

WHEREAS, effective January 1, 2001, the Arch Re Plan was assumed and adopted by the Sponsor and renamed the Arch Capital Group (U.S.) Inc. Executive Supplemental Non-Qualified Savings and Retirement Plan (the “Plan”); and

WHEREAS, the Sponsor and the Trustee wish to amend the Trust Agreement as set forth herein.

NOW, THEREFORE, the Trust Agreement is hereby amended, effective as of the 1^st day of March, 2003, as follows:

1.        The second “WHEREAS” clause of the Trust Agreement is amended to read as follows:

“WHEREAS, the Sponsor wishes to establish an irrevocable trust and to contribute, or cause to be contributed, to the trust assets that shall be held therein, subject to the claims of the creditors of the employer of each Participant (as defined in the Plan) in the event of such employer’s Insolvency, as herein defined, until paid to the Participants and their beneficiaries in such manner and at such times as specified in the Plan; and”

2.        The fourth “WHEREAS” clause of the Trust Agreement is amended to read as follows:

“WHEREAS, it is the intention of the Sponsor to make contributions, or to cause contributions to be made, to the trust to provide a source of funds to assist in the meeting of liabilities under the Plan; and”

3.        Section 1(c) of the Trust Agreement is amended by deleting the last two sentences thereof and replacing them with the following:

“Any rights created under the Plan and this Trust Agreement shall be mere unsecured contractual rights of the Participants and their beneficiaries as set forth in the Plan.  Any assets held by the Trust will be subject to the claims of the creditors of the Sponsor or such other participating employer under the Plan by which the Participant is employed (the “Employer”) under applicable law in the event of Insolvency, as defined in Section 13(a) herein.”

4.        Section 13 of the Trust Agreement is amended to read in its entirety as follows:

“Section 13.  Trustee Responsibility Regarding Payments to Trust Beneficiary When an Employer Is Insolvent.

(a)       The Trustee shall cease payment of benefits to or on behalf of a Participant and his or her beneficiaries if the Participant’s Employer is Insolvent.  An Employer shall be considered “Insolvent” for purposes of this Trust Agreement if (i) the Employer is unable to pay its debts as they become due, or (ii) the Employer is subject to a pending proceeding as a debtor under the United States Bankruptcy Code.

(b)       At all times during the continuance of this Trust, the principal and income of the Trust attributable to a Participant shall be subject to claims of general creditors the Participant’s Employer under applicable law as set forth below.

(i)       The Board of Directors and the Chief Executive Officer of the Sponsor shall have the duty to inform the Trustee in writing of an Employer’s Insolvency.  If a person claiming to be a creditor of an Employer alleges in writing to the Trustee that the Employer has become Insolvent, the Trustee shall determine whether the Employer is Insolvent and, pending such determination, the Trustee shall discontinue payment of benefits to the Participants (or their beneficiaries) who are employed by the Employer.

(ii)       Unless the Trustee has actual knowledge of an Employer’s Insolvency, or has received notice from the Sponsor or a person claiming to be a creditor alleging that an Employer is Insolvent, the Trustee shall have no duty to inquire whether an Employer is Insolvent.  The Trustee may in all events rely on such evidence concerning an Employer’s solvency as may be furnished to the Trustee and that provides the Trustee with a reasonable basis for making a determination concerning the Employer’s solvency.

(iii)      If at any time the Trustee has determined that the Employer of a Participant is Insolvent, the Trustee shall discontinue payment to the Participant or his or her beneficiary and shall hold the assets of the trust attributable to such Participant for the benefit of the Employer’s general creditors.  Nothing in this Trust Agreement shall in any way diminish any rights of the Participants or their beneficiaries to pursue their rights as general creditors of their Employer or Arch Capital Group Ltd. with respect to benefits due under the Plan or otherwise.

(iv)      The Trustee shall resume the payment of benefits to the Participants or their beneficiaries in accordance with this Trust Agreement only after the Trustee has determined that the Participant’s Employer is not Insolvent (or is no longer Insolvent).

(c)       Provided that there are sufficient assets, if the Trustee discontinues the payment of benefits from the Trust pursuant to Section 13(b) hereof and subsequently resumes such payments, the first payment following such discontinuance shall include the aggregate amount of all payments due to the Participants or their beneficiaries under the terms of the Plan for the period of such discontinuance, less the aggregate amount of any payments made to the Participants or their beneficiaries by the Sponsor, another Employer or Arch Capital Group Ltd. in lieu of the payments provided for hereunder during any such period of discontinuance.”

(d)

IN WITNESS WHEREOF, the parties hereto have caused this Agreement to be executed by their duly authorized officers as of the day and year first above written.

AMENDMENT TO THE
Arch Capital Group (US) Inc. Exec. Supplemental Non-Qualified Savings and Retirement Plan

WHEREAS, The Arch Capital Group U.S. Inc. (the “Employer”) adopted the Arch Capital Group (US) Inc. Exec. Supplemental Non-Qualified Savings and Retirement Plan (the “Plan”) through adoption of the Fidelity Investments CORPORATEplan for Retirement^SM Select Plan Basic Plan Document (“the Basic Plan Document”), effective as of  January 1, 1996, for which Fidelity Management Trust Company (the “Trustee”) serves as trustee of the trust established to hold Plan assets (the “Trust”); and

WHEREAS, the Employer desires to amend the Plan to remove the choice of investment options from the Adoption Agreement:

NOW THEREFORE, the Employer amends the Plan as follows effective 9/1/2005.

1.   Section 1.11(b) is amended to replace all language between the first sentence and the first “Note” with the following sentence:

Participant Accounts under the Trust will be invested among the Permissible Investments designated in the Service Agreement.

2.   Section 2.01(a)(20) is amended to read as follows:

“Registered Investment Company” means any one or more corporations, partnerships or trusts registered under the Investment Company Act of 1940.

3.   Section 2.01(a)(28) is added to the Basic Plan Document as follows:

“Permissible Investment” means the investments specified by the Employer as available for investment of assets of the Trust and agreed to by the Trustee.

4.   Section 2.01(a)(29) is added to the Basic Plan Document as follows:

“Service Agreement” means the agreement between the Employer and the Trustee relating to the provision of investment and other services to the Plan and shall include any addendum to the agreement and any other separate written agreement between the Employer and the Trustee relating to the provision of services to the Plan.

5.   All occurrences of “Section 1.11(b)” in the Basic Plan Document will be replaced with “the Service Agreement”.

In witness whereof, the Employer has signed this instrument this 10th day of August, 2005.

Amendment To The CORPORATEplan for Retirement^SM Select

Service Agreement – Mutual Fund Windows

WHEREAS, Arch Capital Group U.S. Inc.  (the “Employer”) and Fidelity Management Trust Company as the trustee (“Trustee”) executed the Fidelity CORPORATEplan for Retirement^SM Select Service Agreement (“CPR Service Agreement”) for the Arch Capital Group (US) Inc. Exec. Supplemental Non-Qualified Savings and Retirement Plan (Fidelity Plan # 44023), and

WHEREAS, Article V, Section 9, of the CPR Service Agreement provides that it may be amended by a written agreement signed by the Employer and the Trustee; and

NOW THEREFORE, the Employer and Trustee hereby agree to the following:

1.   Notwithstanding anything else in the CPR Service Agreement to the contrary in the CPR Service Agreement, investments made available to the Plan for investment of assets of the Trust (herein called “Permissible Investments”) shall be listed in the CPR Service Agreement.

2.   Notwithstanding anything else in the CPR Service Agreement to the contrary in the CPR Service Agreement, Fidelity shall also be entitled to any fees relating to Permissible Investments stated in Article V of the CPR Service Agreement or any amendment thereto.

3.   The following shall be added as a separate section following the last section of Article V:

Mutual Funds Available:       The Employer has selected certain mutual funds as Permissible Investments for investments of Participant Accounts under the Trust.  Unless specifically indicated otherwise within this Agreement or an amendment to this Agreement, purchases, sales and exchanges of each Permissible Investment option are controlled by that Permissible Investment’s prospectus or other governing document(s).  The Employer and Fidelity have agreed that the following constraints, limitations, fees and operating procedures shall apply in reference to the listed Permissible Investments:

II.       FIDELITY FUNDS (CORE)

III.       NON-FIDELITY FUNDS (CORE)

Fidelity shall provide recordkeeping services for Non-Fidelity Funds subject to and in accordance with the terms and conditions of this Section:

1.   For purposes of this Agreement, ‘Non-Fidelity Fund’ shall mean an investment company registered under the Investment Company Act of 1940, as amended, other than one advised by Fidelity Management & Research Company, and specified in an agreement between Fidelity and the transfer agent for such investment company (‘Fund Vendor’).

2.   The basis-point-per-annum fee charged by Fidelity shall be computed and billed or charged in arrears quarterly based on the market value of Non-Fidelity Funds held in Participant Accounts on the last business day of the quarter.  In addition to the fees specified above, Fidelity shall be entitled to fees from the Fund Vendor as set forth in a separate agency agreement with the Fund Vendor. Fidelity will make available appropriate information concerning the current provisions of such agreements electronically (currently through Plan Sponsor Webstation) for the Employer’s review.

3.   The Fund Vendor shall prepare and provide descriptive information on the funds for use by Fidelity in its written participant communication materials.  Fidelity shall utilize historical performance data obtained from third-party vendors in communications with plan participants.  The Employer hereby consents to Fidelity’s use of such materials and acknowledges that Fidelity is not responsible for the accuracy of such third-party information.

For all Non-Fidelity Funds a Basis-point-per-annum fee of $0 will be charged by Fidelity on amounts invested in the Non-Fidelity Fund.

That Basis-point-per-annum fee is paid by the Fee Waived.

Annual Fee for Excess Core Permissible Investment Options

The fees stated in this Service Agreement take into consideration the Permissible Investment options selected by the Employer in this Service Agreement and include up to 30 Core Permissible Investment options with no additional annual fee.  The annual fee for each Core Permissible Investment option in excess of 30 is $500 per option and such fee is in addition to any fees specified elsewhere in this Service Agreement, including any Appendices and amendments hereto. The annual fee for excess Core Permissible Investment options shall be billed or charged quarterly in arrears and paid by the Employer. The Fidelity Freedom funds collectively shall each count as one Core Permissible Investment option. Any change to the Permissible Investment options selected by the Employer after the effective date of this Service Agreement shall require an amendment to this Service Agreement and may result in amended or additional fees.

Mutual Fund Window

Participants may invest through a Mutual Fund Window arrangement under the Plan, hereinafter referred to as “MFW”.  Fidelity shall provide recordkeeping services for MFW in accordance with the terms and conditions this Section.

a.   This MFW allows a participant to invest in any of the Fidelity Funds listed below.

b.   The quarterly per participant fee will be assessed for every participant having a balance at the end of the billing period in any fund available only through this MFW.  Participants may invest in any of the Permissible Investments described outside this MFW paragraph (hereinafter “Core Funds”) without being subject to these MFW fees.  The annual fee for excess permissible investment options listed in this Agreement shall only apply to those selected Core Funds.

c.   The following is a list of all the funds in which a Plan Participant may invest through MFW (some Core Funds may appear in this list as well, but those will not be treated as available only through this MFW):

Fidelity Funds (in alphabetical order):

Non-Fidelity Funds (in alphabetical order):

For all Non-Fidelity Funds a Basis-point-per-annum fee of $0 will be charged by Fidelity on amounts invested in the Non-Fidelity Fund.

That Basis-point-per-annum fee is paid by the  Not Applicable.

Fidelity shall provide recordkeeping services for Non-Fidelity Funds subject to and in accordance with the terms and conditions of this Section:

1.   For purposes of this Agreement, ‘Non-Fidelity Fund’ shall mean an investment company registered under the Investment Company Act of 1940, as amended, other than one advised by Fidelity Management & Research Company, and specified in an agreement between Fidelity and the transfer agent for such investment company (‘Fund Vendor’).

2.   The basis-point-per-annum fee charged by Fidelity shall be computed and billed or charged in arrears quarterly based on the market value of Non-Fidelity Funds held in Participant Accounts on the last business day of the quarter.  In addition to the fees specified above, Fidelity shall be entitled to fees from the Fund Vendor as set forth in a separate agency agreement with the Fund Vendor. Fidelity will make available appropriate information concerning the current provisions of such agreements electronically (currently through Plan Sponsor Webstation) for the Employer’s review.

3.   The Fund Vendor shall prepare and provide descriptive information on the funds for use by Fidelity in its written participant communication materials.  Fidelity shall utilize

historical performance data obtained from third-party vendors in communications with plan participants.  The Employer hereby consents to Fidelity’s use of such materials and acknowledges that Fidelity is not responsible for the accuracy of such third-party information.

The Employer understands that this MFW service is an investment selection of a certain group of funds currently available for the Plan and that the funds present in this MFW service change over time.  The Employer understands that a choice can be made at any time to change from the MFW service to another investment platform (another window or a platform without a window investment) offered by Fidelity and available to the Plan.  The Employer will always have the option to move to an investment platform of 15 Fidelity Funds (not Select Funds) chosen from among those available to the Plan.  The Employer agrees that any change of investment platform will be effective as soon as administratively feasible for Fidelity (after the Employer and Fidelity have amended this agreement to reflect such change) and that the Employer will communicate to participants the date and consequences of such change.

The Employer hereby directs Fidelity to add new funds to the Permissible Investments for the Plan as those funds are added to MFW service.  Fidelity shall always give the Employer at least 90 days notice of the date that new fund(s) will become available through the MFW service and the Employer has until 20 days before such date to direct Fidelity not to make any such new fund or funds available for the Plan.  The Employer understands that, since this service is a package service and Fidelity is unable to customize this package, the Employer’s decision not to add certain fund(s) may mean that the Plan is unable to remain on its current MFW service.  If the Employer’s decisions make the Plan unable to remain on its current MFW service, the Employer agrees to choose an entirely different investment platform (another MFW service or a platform without a MFW service) for the Plan at least 10 business days before such new funds are to be added to the current MFW service.

The Employer hereby directs Fidelity to remove from the Permissible Investments for the Plan any funds being removed as a fund available under the MFW service (“Non-MFW Funds”).  Fidelity shall always give the Employer at least 90 days notice of the date that a fund or funds will become Non-MFW Funds.  If the Employer decides to change any of the Core Permissible Investments for the Plan, the Employer agrees that Fidelity may reassess the Plan’s fee structure and charge additional fees based upon the Core Permissible Investments for the Plan resulting from the Employer’s decision.  The Employer hereby directs Fidelity to exchange all balances present in the any such Non-MFW funds, on the date such a Non-MFW Fund ceases to be a Permissible Investment for the Plan, into the default investment for the Plan.

The Employer agrees that any closure of a fund that is part of the MFW service will be treated as a fund being removed from the MFW service pursuant to the paragraph immediately preceding, except that Fidelity may adjust notice timeframes as circumstances dictate.  The Employer further agrees that anytime a fund within the MFW service merges completely (thus ceasing to exist) into a fund that is not currently a Permissible Investment under the Plan, that such fund will be treated as if it was closing pursuant to the previous sentence.  Fidelity agrees that anytime two funds that are Permissible Investments under the Plan merge, Fidelity will assist the Employer with communicating in advance the consequences of the merger to its Plan

participants.  The Employer agrees that when two Permissible Investments merge Plan participants who do not act before the date of the merger will have their account balances in each such fund combined in the resulting fund.  If a merger of two Permissible Investments will result in the Plan having one less Core Permissible Investment, Fidelity agrees to give the Employer the opportunity to remove the resulting fund from the MFW service and assign it as a Core Permissible Investment.  The Employer understands that the timing of the merger of funds is outside of the control of the Trustee and that all assistance to be provided the Employer will always be on a best efforts basis.

AND FURTHER AGREE, that this Amendment shall be effective upon the date indicated below and that the changes made by this Amendment are incorporated into the Service Agreement and control over conflicting provisions of any previously executed Service Agreement, or any amendment or addendum thereto.

This Amendment shall be effective 9-1, 2005.

The CORPORATE Plan for Retirement

Select Plan

Trust Agreement Amendment

Pursuant to Section 14 of the Trust Agreement, the below-signed parties hereby agree to amend the Trust Agreement to provide, in conjunction with amendments to the Adoption Agreement and the Service Agreement, that the Trustee will make distributions from the Plan directly to Plan Participants and do certain tax withholding and reporting as follows:

Replace in its entirety subsection (a) of Section 3, Disbursements, with the following:

(a)  Directions from Administrator.

The Trustee shall disburse monies to employee participants and their beneficiaries for benefit payments in the amounts that the Administrator directs from time to time in writing.  The Trustee shall have no responsibility to ascertain whether the Administrator’s direction complies with the terms of the Plan or of any applicable law.  The Trustee shall be responsible for any Federal or State income tax reporting or withholding with respect to such Plan distributions.  The Trustee shall not be responsible for the calculation, withholding or reporting of any FICA, FUTA, Social Security, Medicare, State unemployment, or local income taxes with respect to Plan distributions.

Replace in its entirety subsection (e) of Section 5, Recordkeeping and Administrative Services to Be Performed, with the following:

(e)  Returns, Reports and Information:

Except as provided in Section 5(a) of this Agreement, the Administrator shall be responsible for the preparation and filing of all returns, reports, and information required of the Trust or Plan by law including but not limited to any annual fiduciary tax return.  The Trustee shall provide the Administrator with such information as the Administrator may reasonably request to make these filings.  The Administrator shall also be responsible for making any disclosures to participants required by law.