<PAGE>   1
                                                                     Exhibit 4.6

                            FOURTH AMENDMENT TO THE
                 INFORMATION AND REGISTRATION RIGHTS AGREEMENT

        This Fourth Amendment to the Information and Registration Rights
Agreement, as amended (the "Amendment"), is made as of September 1, 1998, by and
among Biosource Technologies, Inc., a California corporation ("Biosource"), and
The Dow Chemical Company ("TDCC").

                                 R E C I T A L S

        A. Biosource and TDCC have entered into a Common Stock Warrant Agreement
dated of even date herewith (the "Warrant Agreement") whereby Biosource has
agreed to grant to TDCC a warrant to purchase Common Stock of Biosource (the
"Warrant").

        B. Biosource and TDCC desire to provide for the rights of TDCC with
respect to registration of the shares of the Common Stock purchased by TDCC
pursuant to TDCC's exercise of the Warrant according to the terms of the
Information and Registration Rights Agreement dated as of October 11, 1990, as
amended by the Amendment to the Information and Registration Rights Agreement
dated as of October 11, 1990, the Second Amendment to the Information and
Registration Rights Agreement dated as of October 10, 1991 and the Third
Amendment to the Information and Registration Rights Agreement dated as of March
20, 1998 (collectively, the "Agreement").

        C. Biosource and TDCC desire to amend the Agreement to grant to TDCC
Registration rights in accordance with the Agreement with respect to the shares
of Common Stock purchased by TDCC pursuant to the exercise of the Warrants
purchased by TDCC pursuant to the Warrant Agreement.



<PAGE>   2

        THE PARTIES AGREE AS FOLLOWS:

        1. Definitions. Unless otherwise indicated, the capitalized terms used
in this Amendment without definition shall have the same meanings as set forth
in the Agreement.

        2. Grant of Registration Rights. In accordance with Section 9 of the
Agreement, TDCC shall be considered an Investor for purposes of the Agreement
with respect to shares of Common Stock purchased by TDCC pursuant to exercise of
the Warrant.

        3. Counterparts. This Amendment may be executed in two or more
counterparts, each of which shall be deemed an original, but all of which
together shall constitute one and the same instrument.

        4. Governing Law. This Amendment shall be governed by and construed in
accordance with the laws of the state of California applicable to contracts
entered into and wholly to be performed within the state of California.

        5. Headings. The headings of the Sections of this Amendment are for
convenience and shall not by themselves determine the interpretation of this
Agreement.


                                       2
<PAGE>   3

        IN WITNESS WHEREOF, the parties hereto have executed this Amendment as
of the day and year first above written.

BIOSOURCE TECHNOLOGIES, INC.             THE DOW CHEMICAL COMPANY



By: /s/ Robert L. Erwin                  By:
   ----------------------------------       ------------------------------------
   Robert L. Erwin, Chairman of the      Name:
   Board and Chief Executive Officer          ----------------------------------
                                         Title:
                                               ---------------------------------






             Signature Page for Fourth Amendment to the Information
                        and Registration Rights Agreement