<PAGE>   1



                                                                    EXHIBIT 4.21




                                FIRST SUPPLEMENT

                                       to

                               PURCHASE CONTRACTS

                                     between

                         AMERICAN MUTUAL HOLDING COMPANY

                                       and

                                    HOLDERS,
                                  as specified












                            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 HOLDERS,
as specified on the Unit Certificates (the "Holders").

     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, the Company has entered into a Purchase Contract with each Holder,
as evidenced by each Unit Certificate (the "Purchase Contracts"), which
obligates the Holder to purchase, and the Company to sell, Common Stock on the
Stock Purchase Date;

     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, 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.; and

     WHEREAS, AMHC wishes to assume the obligations of the Company under the
Purchase Contracts, pursuant to the requirements of Article Nine 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.      Capitalized terms not otherwise defined herein shall have
                the meanings set forth in the Master Unit Agreement and the
                Purchase Contracts.

Section 2.      AMHC hereby agrees to assume the obligations of the
                Company, as its successor, under the Purchase Contracts and to
                be bound by the provisions of such \ agreements.

Section 3.      This First Supplemental Agreement shall become effective as
                of the Effective Time (as defined in the Agreement and Plan of
                Merger referred to above) of the Merger.

                                       2



<PAGE>   3


Section 4.      This First 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 5.      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



                                    HOLDERS

                                    By:  First Union National Bank,
                                          as Attorney-in-Fact of such Holders



                                    By:  /s/ Shawn K. Bednasek
                                    -------------------------------------------
                                    Name:  Shawn K. Bednasek
                                    Title: Vice President




                                       3