Document:

<PAGE>   1
                                                                  EXHIBIT 4.18

                                FIRST SUPPLEMENT

                                       to

                              MASTER UNIT AGREEMENT

                                     between

                         AMERICAN MUTUAL HOLDING COMPANY

                                       and

                    FIRST UNION NATIONAL BANK, as Unit Agent

                            Dated September 20, 2000

<PAGE>   2

                          FIRST SUPPLEMENTAL AGREEMENT

     This First Supplemental Agreement (this "First Supplemental Agreement"),
dated as of September 20, 2000, is among AMERICAN MUTUAL HOLDING COMPANY, an
Iowa mutual insurance holding company ("AMHC"), as successor to AMERUS LIFE
HOLDINGS, INC., an Iowa insurance holding company (the "Company"), and FIRST
UNION NATIONAL BANK, a national banking corporation, acting as unit agent for
the Holders of Units from time to time (the "Unit Agent").

     WHEREAS, the Company and the Unit Agent entered into a Master Unit
Agreement, dated as of July 27, 1998 (the "Master Unit Agreement"), in which the
obligations of the Company under the Units are provided;

     WHEREAS, AMHC has executed a Plan of Conversion, dated as of December 17,
1999, pursuant to which AMHC will convert from a mutual insurance holding
company into a stock holding company;

     WHEREAS, the Company and AMHC have entered into an Agreement and Plan of
Merger, dated December 17, 1999 (the "Merger Agreement"), pursuant to which the
Company will merge with and into AMHC, with AMHC continuing as the surviving
company (the "Merger") and AMHC will thereafter change its name to AmerUs Group
Co.;

     WHEREAS, AMHC wishes to assume the obligations of the Company under the
Master Unit Agreement, as authorized by Article Eight of the Master Unit
Agreement and without the consent of the Holders; and

     WHEREAS, AMHC wishes to provide the rights provided by Section 506 of the
Master Unit Agreement to the Holders of each Outstanding Unit, pursuant to the
requirements of Section 5.06(b) of the Master Unit Agreement, as authorized by
Article Eight of the Master Unit agreement and without the consent of the
Holders.

     NOW THEREFORE, in consideration of the foregoing, the parties agree as
follows:

Section 1.  Definitions.  Capitalized terms not otherwise defined herein shall
            have the meanings set forth in the Master Unit Agreement.

Section 2.  Assumption.  AMHC hereby agrees to assume the obligations of the
            Company, as its successor, under the Master Unit Agreement and to
            be bound by the provisions of such agreements.

Section 3.  Adjustment of Settlement Rate. The Holders of each Outstanding
            Unit shall have the rights provided by Section 506 of the Master
            Unit Agreement and adjustment to the Settlement Rate will be made
            accordingly.

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Section 4.  Subsequent Adjustment of Settlement Rate. Adjustments to the
            Settlement Rate for events that occur subsequent to the Effective
            Time (as defined in the Merger Agreement) of the Merger shall be
            as nearly equivalent as may be practicable to adjustments provided
            for in Section 506.

Section 5.  Notice.  Section 1.05 of the Master Unit Agreement will be amended
            to delete the Trustee's notice address and insert the following
            notice in lieu thereof:

                           401 South Tryon Street
                           12th Floor
                           Charlotte, NC 28288-1179

Section 6.  Effectiveness.  This Supplemental Agreement shall become effective
            as of the Effective Time (as defined in the Merger Agreement) of
            the Merger.

Section 7.  Counterparts.  This Supplemental Agreement may be executed in two
            or more counterparts, each of which will be deemed to be an
            original, but all of which together will constitute one and the
            same agreement.

Section 8.  Governing Law.  This Supplemental Agreement will be governed by and
            interpreted in accordance with the internal laws of the State of
            New York, without giving effect to principles of conflicts of laws.

            IN WITNESS WHEREOF, the parties hereto have executed and delivered
this Supplemental Agreement as of the date set forth above.

                                    AMERICAN MUTUAL HOLDING COMPANY

                                    By: /s/ Roger K. Brooks
                                        ---------------------------------------
                                          Name:  Roger K. Brooks
                                          Title: Chairman, President and
                                                 Chief Executive Officer

                                    FIRST UNION NATIONAL BANK, as Unit Agent

                                    By: /s/ Shawn K. Bednasek
                                        ---------------------------------------
                                          Name:  Shawn K. Bednasek
                                          Title: Vice President<PAGE>   1

                                                                  EXHIBIT 4.19

                       ASSIGNMENT AND ASSUMPTION AGREEMENT

                                       to

                            QUIPS GUARANTEE AGREEMENT

                                     between

                         AMERICAN MUTUAL HOLDING COMPANY

                                       and

                           AMERUS LIFE HOLDINGS, INC.

                            Dated September 20, 2000

<PAGE>   2

                       ASSIGNMENT AND ASSUMPTION AGREEMENT

     This ASSIGNMENT AND ASSUMPTION AGREEMENT ("Agreement") is made and entered
into as of the 20th day of September, 2000, between AMERICAN MUTUAL HOLDING
COMPANY, an Iowa mutual insurance holding company ("AMHC") and AMERUS LIFE
HOLDINGS, INC., an Iowa insurance holding company ("AmerUs").

                                   WITNESSETH:

     WHEREAS, AmerUs Capital II (the "Trust") was established as a business
trust under the Delaware Business Trust Act pursuant to a Trust Agreement, dated
as of April 14, 1998 (the "Original Declaration"), between AmerUs and First
Union Trust Company, National Association, as Trustee, and a Certificate of
Trust executed and filed with the Secretary of State of the State of Delaware on
April 14, 1998, for the sole purpose of issuing and selling certain securities
representing undivided beneficial interests in the assets of the Trust and
investing the proceeds thereof in certain Debentures (as defined in the
Declaration) of AmerUs;

     WHEREAS, the Original Declaration was amended and restated, as of July 27,
1998 (the "Declaration"), by AmerUs, the trustees of the Trust, the
administrators of the Trust, and the holders of undivided beneficial interests
in the assets of the Trust to be issued from time to time, to provide for the
operation of the Trust and the issuance of Capital Securities and Common
Securities;

     WHEREAS, AmerUs and First Union National Bank, a national banking
corporation, as trustee, for the benefit of the Holders from time to time of the
Capital Securities of the Trust, entered into a QUIPS Guarantee Agreement, dated
as of July 27, 1998 (the "QUIPS Guarantee Agreement"), in which the Guarantor
agreed to pay to the Holders the Guarantee Payments and to make certain other
payments on the terms and conditions set forth in the QUIPS Guarantee Agreement;

     WHEREAS, AMHC has executed a Plan of Conversion, dated as of December 17,
1999, pursuant to which AMHC will convert from a mutual insurance holding
company into a stock holding company;

     WHEREAS, AMHC and the Guarantor entered into an Agreement and Plan of
Merger, dated December 17, 1999, pursuant to which the Guarantor will merge with
and into AMHC, with AMHC as the surviving company (the "Merger") and AMHC will
thereafter change its name to AmerUs Group Co.;

     WHEREAS, AMHC desires to assume the Guarantor's obligations under the QUIPS
Guarantee Agreement;

     WHEREAS, this Assumption Agreement is being executed pursuant to Section
10.01 of the QUIPS Guarantee Agreement; and

     WHEREAS, the Merger is permitted under Article Seven of the Indenture.

<PAGE>   3

     NOW, THEREFORE, the parties agree as follows:

SECTION 1.  Assignment.  Upon the execution of this Agreement by the parties
            hereto, AmerUs does hereby assign and transfer all of its
            obligations under the QUIPS Guarantee Agreement to AMHC, as its
            successor.

SECTION 2.  Assumption. Upon the execution of this Agreement by the parties
            hereto, AMHC does hereby absolutely and irrevocably accept the
            foregoing assignment and hereby assumes to be solely responsible
            for and to perform, as successor, AmerUs' obligations under the
            QUIPS Guarantee Agreement.

SECTION 3.  Definitions. For all purposes of this Agreement, except as
            otherwise herein expressly provided, the definitions, terms
            and expressions used herein shall have the same meanings as
            corresponding definitions, terms and expressions used in the
            QUIPS Guarantee Agreement.

SECTION 4.  Effectiveness.  This Agreement shall become effective as of the
            Effective Time (as defined in the Agreement and Plan of Merger
            referred to above) of the Merger.

SECTION 5.  Binding Effect; Assigns.  This Agreement shall be binding upon,
            and shall enure to the benefit of, the parties thereto and their
            respective successors and assigns.

SECTION 6.  Counterparts.  This Agreement may be executed in two or more
            counterparts, each of which will be deemed to be an original, but
            all of which together will constitute one and the same agreement.

SECTION 7.  Governing Law.  This Agreement will be governed by and interpreted
            in accordance with the internal laws of the State of New York,
            without giving effect to principles of conflicts of laws.

            IN WITNESS WHEREOF, the parties hereto have executed and
delivered this Assumption Agreement as of the date set forth above.

                                        AMERICAN MUTUAL HOLDING COMPANY

                                        By: /s/ Roger K. Brooks
                                               --------------------------------
                                            Name:  Roger K. Brooks
                                            Title: Chairman, President and
                                                   Chief Executive Officer

<PAGE>   4

                                        AMERUS LIFE HOLDINGS, INC.

                                        By: /s/ Roger K. Brooks
                                            -----------------------------------
                                            Name:  Roger K. Brooks
                                            Title: Chairman, President and
                                                   Chief Executive Officer

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