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                                                                     Exhibit 4.6

                               FOURTH AMENDMENT TO
                     COMMON STOCK PURCHASE WARRANT AGREEMENT

This Fourth Amendment to Common Stock Purchase Warrant Agreement, dated as of
October X, 2001, is between MURDOCK COMMUNICATIONS CORPORATION, an Iowa
corporation, (the "Company") and FIRSTAR BANK, N.A., as Warrant Agent and
successor-in-interest to Firstar Trust Company (the "Warrant Agent").

                                    RECITALS

     A.   The Warrant Agent and the Company are parties to a Common Stock
          Purchase Warrant Agreement dated as of October 21, 1996 (the
          "Agreement").

     B.   The Warrant Agent and Company entered into a First Amendment to
          Warrant Agreement dated as of September 30, 1999, to, among other
          things, extend the "Warrant Expiration Date" to April 21, 2000, a
          Second Amendment to Warrant Agreement dated as of April 14, 2000, to
          extend the "Warrant Expiration Date" to October 21, 2000 and a Third
          Amendment to Warrant Agreement dated as of October 9, 2000, to extend
          the "Warrant Expiration Date" to October 21, 2001.

     C.   The Company and Warrant Agent deem it desirable to amend the Agreement
          to extend the "Warrant Expiration Date" to October 21, 2002.

     D.   Pursuant to Section 10 of the Agreement, the Company and the Warrant
          Agent may by supplemental agreement make any changes or corrections in
          the Agreement that they may deem necessary or desirable and which do
          not adversely affect the interests of the holders of Warrant
          Certificates.

                                   AGREEMENTS

     In consideration of the foregoing and the mutual covenants and agreements
     contained herein and in the Agreement, and intending to be legally bound
     hereby, the Warrant Agent and Company hereby agree as follows:

     1.   Amendment of Section 1 (i). The definition of "Warrant Expiration
          Date" is Section 1 (i) of the Agreement is hereby amended to read in
          its entirety as follows:

          "Warrant Expiration Date" shall mean 5 p.m. (Central Time) on October
          21, 2002, or if such date shall in the State of Wisconsin be a holiday
          or a day on which banks are authorized to closed, then 5 p.m. (Central
          Time) on the next following day which in the State of Wisconsin is not
          a holiday or a day on which banks are authorized to close. Unless
          exercised during the Warrant Exercise Period, the Warrants will
          automatically expire. The Warrants may be called for redemption and
          the expiration date herefor accelerated, on the terms and conditions
          set forth in sections 4(b) and 4(c) of this Agreement. If so called
          for redemption, Warrant Certificate holders shall have a period of at
          least 30 days after the date of the call notice within which to
          exercise the Warrants. However, Warrant Certificate holders will
          receive the redemption price only if such certificates are surrendered
          to the Corporate Office with the redemption period (as defined below).

     2.   Full Force and Effect. All remaining provisions of the Agreement, as
          previously amended, remain unchanged and in full force and effect.

     3.   Governing Law. This Amendment shall be governed by and construed in
          accordance with the laws of the State of Wisconsin. Section headings
          in this Amendment appear for convenience of reference only and shall
          not be used in any interpretation of this Amendment.

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     4.   Binding Effect. This Amendment shall be binding upon and insure to the
          benefit of the Company, the Warrant Agent and their respective
          successors and assigns, and the Registered Holders from time to time
          of Warrant Certificates or any of them. Nothing in this Amendment
          shall be construed to confer any right, remedy or claim upon any other
          person.

     5.   Counterparts. This Amendment may be executed in counterparts, which
          taken together shall constitute a single document.

     IN WITNESS WHEREOF, the Warrant Agent and the Company have caused this
     Amendment to be executed as of the date first written above by their
     respective officers thereunto duly authorized.

                                           MURDOCK COMMUNICATIONS CORPORATION


                                           BY /s/ Paul C. Tunink
                                              ----------------------------------
                                                  Paul C. Tunink, Vice President


                                           FIRSTAR BANK, N.A.


                                           BY /s/
                                              ----------------------------------
                                                  Authorized Officer