<PAGE>

                                                                    EXHIBIT 4.3

                                                                  EXECUTION COPY



                                  VIACOM INC.,

                            VIACOM INTERNATIONAL INC.

                                       AND

                                 CITIBANK, N.A.,

                                     Trustee

                  --------------------------------------------


                          FOURTH SUPPLEMENTAL INDENTURE

                           Dated as of August 1, 2000



                      To Indenture dated as of May 15, 1995
                                      among
                                  VIACOM INC.,
                            VIACOM INTERNATIONAL INC.
                                       and
                                 CITIBANK, N.A.,
                                     Trustee

                  --------------------------------------------


                             Senior Debt Securities

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            FOURTH SUPPLEMENTAL INDENTURE, dated as of August 1, 2000, among
VIACOM INC., a Delaware corporation (the "COMPANY"), VIACOM INTERNATIONAL INC.,
a Delaware corporation (the "GUARANTOR"), and CITIBANK, N.A., a national banking
association, as successor in interest to State Street Bank and Trust Company and
The First National Bank of Boston, trustee under the Indenture (the "TRUSTEE"),
dated as of May 15, 1995, among the Company, the Guarantor and the trustee party
thereto, as supplemented by the First Supplemental Indenture, dated as of May
24, 1995, among the Company, the Guarantor and the trustee party thereto, as
supplemented and amended by the Second Supplemental Indenture and Amendment No.
1, dated as of December 15, 1995, among the Company, the Guarantor and the
trustee party thereto, and as further supplemented by the Third Supplemental
Indenture, dated as of July 22, 1996, among the Company, the Guarantor and the
trustee party thereto (as so amended, the "INDENTURE").

                             RECITALS OF THE COMPANY

            WHEREAS, Section 901(1) of the Indenture permits supplements thereto
without the consent of Holders of Securities to add to the covenants of the
Company for the benefit of all or any series of Securities;

            WHEREAS, Section 901(4) of the Indenture permits supplements thereto
without the consent of Holders of Securities to change any provisions of the
Indenture with respect to a series of Securities, where there are no Securities
Outstanding which are entitled to the benefit of such provision;

            WHEREAS, as contemplated by Section 301 of the Indenture, the
Company intends to issue and the Guarantor intends to guarantee from time to
time two new series of Securities, consisting of the 7.70% Senior Notes due 2010
(the "SENIOR NOTES") and the 7.875% Senior Debentures due 2030 (the "SENIOR
DEBENTURES") under the Indenture; and

            WHEREAS, the Company wishes to add to the covenants of the Company
for the sole benefit of the Holders of the Senior Notes and the Senior
Debentures and to make certain changes to other provisions of the Indenture, as
contemplated by Section 301 of the Indenture.

            NOW, THEREFORE, THIS FOURTH SUPPLEMENTAL INDENTURE WITNESSETH:

SECTION 1. For the purpose of this Fourth Supplemental Indenture, all terms used
herein, unless otherwise defined, shall have the meaning assigned to them in the
Indenture.

SECTION 2. The Company shall issue from time to time, and the Guarantor shall
guarantee from time to time, (a) Senior Notes, in the form of EXHIBIT A attached
hereto, and (b) Senior Debentures, in the form of EXHIBIT B attached hereto. The
Senior Notes will be initially issued in an aggregate principal amount of
$1,150,000,000 and the Senior Debentures will be initially issued in an
aggregate principal amount of $500,000,000.

SECTION 3. For the sole benefit of the Holders of the Senior Notes and the
Senior Debentures, Section 101 of the Indenture shall be amended by deleting the
definitions of "Authorized

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Newspaper," "Corporate Trust Office," "Principal Property," "Sale and Leaseback
Transaction"and "United States Person" and replacing such definitions with the
following definitions:

                  "AUTHORIZED NEWSPAPER" means a newspaper, in the English
      language or in an official language of the country of publication,
      customarily published on each Business Day, whether or not published on
      Saturdays, Sundays or holidays, and of general circulation in the place in
      connection with which the term is used or in the financial community of
      such place; PROVIDED, HOWEVER, that so long as a series of Securities is
      listed on the Luxembourg Stock Exchange, Authorized Newspaper solely with
      respect to such series of Securities shall mean a leading newspaper having
      general circulation in Luxembourg (which is expected to be the LUXEMBOURG
      WORT) or, if publication in such newspaper is not practicable, a leading
      English language daily newspaper with general circulation in Europe, that
      is published each Business Day in morning editions, whether or not
      published in Saturday, Sunday or holiday editions. Where successive
      publications are required to be made in Authorized Newspapers, the
      successive publications may be made in the same or in different newspapers
      in the same city meeting the foregoing requirements and in each case on
      any Business Day.

                  "CORPORATE TRUST OFFICE" means the office of the Trustee at
      which the corporate trust business of the Trustee shall, at any particular
      time, be principally administered, which office is, at the date of this
      Indenture, located at Citibank, N.A., 111 Wall Street, 14th Floor, New
      York, New York, 10005; Attention: Citibank Agency and Trust Services.

                  "PRINCIPAL PROPERTY" means any parcel of real property and
      related fixtures or improvements (other than telecommunications equipment,
      including, without limitation, satellite transponders) owned by the
      Company or any Restricted Subsidiary and located in the United States, the
      aggregate book value of which on the date of determination exceeds $1.0
      billion, other than any such real property and related fixtures or
      improvements which, as determined in good faith by the Board of Directors
      of the Company, is not of material importance to the total business
      conducted by the Company and its Subsidiaries, taken as a whole.

                  "SALE AND LEASEBACK TRANSACTION" means any arrangement with
      any Person providing for the leasing by the Company or any Restricted
      Subsidiary of any Principal Property which has been or is to be sold or
      transferred by the Company or such Person; PROVIDED, HOWEVER, that "Sale
      and Leaseback Transaction" shall not include such arrangements that were
      existing on August 1, 2000, or at the time any Person owning a Principal
      Property becomes a Restricted Subsidiary (whether by acquisition or
      otherwise, including through merger or consolidation).

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                  "UNITED STATES PERSON" means any individual who is a citizen
      or resident of the United States, a corporation, partnership or other
      entity created or organized in or under the laws of the United States, any
      state thereof or the District of Columbia (other than a partnership that
      is not treated as a United States Person under any applicable Treasury
      regulations), any estate the income of which is subject to United States
      federal income taxation regardless of its source, or any trust if a court
      within the United States is able to exercise primary supervision over the
      administration of the trust and one or more United States Persons have the
      authority to control all substantial decisions of the trust.
      Notwithstanding the preceding sentence, to the extent provided in the
      Treasury regulations, certain trusts in existence on August 20, 1996, and
      treated as United States Persons prior to such date that elect to continue
      to be treated as United States Persons, will also be United States
      Persons.

SECTION 4. For the sole benefit of the Holders of the Senior Notes and Senior
Debentures, Section 101 of the Indenture shall be amended by adding the
following definitions, each in appropriate alphabetical order:

                  "AGENT" means any Registrar, co-Registrar, Paying
      Agent or Authenticating Agent.

                  "DEPOSITARY" means, as applicable, either the U.S.
      Depositary or the Common Depositary.

                   "MAKE-WHOLE AMOUNT" means, with respect to a series of
      Securities, the excess, if any, of (i) the aggregate present value as of
      the Redemption Date of principal being redeemed and the amount of interest
      (exclusive of interest accrued to the Redemption Date) that would have
      been payable if redemption had not been made, determined by discounting,
      on a semiannual basis, the remaining principal and interest payment for
      such series of Securities at the Reinvestment Rate (determined on the
      third Business Day preceding the date notice of redemption shall have been
      given) from the dates on which the principal and interest would have been
      payable with respect to such series of Securities if the redemption had
      not been made, to the applicable Redemption Date, over (ii) the aggregate
      principal amount of the applicable series of Securities being redeemed.

                  "NON-UNITED STATES PERSON" means a Person who is not
      a United States Person.

                  "PARTICIPANT" means, a Person who has an account with
      a Depositary.

                  "REINVESTMENT RATE" means 0.25% for the series of Securities
      designated as 7.70% Senior Notes due 2010, and means 0.35% for the series
      of Securities designated as 7.875% Senior Debentures due 2030, in each
      case plus

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      the arithmetic mean of the yields under the heading "Week Ending"
      published in the most recent Federal Reserve Statistical Release H.15 (or
      any comparable successor publication) under the caption "Treasury Constant
      Maturities" for the maturity (rounded to the nearest month) corresponding
      to the remaining life to maturity, as of the Maturity of the principal of
      the series of Securities being redeemed or paid. If no maturity exactly
      corresponds to the Maturity of such series of Securities, yields for the
      two published maturities most closely corresponding to the Maturity of
      such series of Securities would be so calculated and the Reinvestment Rate
      would be interpolated or extrapolated on a straight-line basis, rounding
      to the nearest month. The most recent Statistical Release published prior
      to the date of determination of the Make-Whole Amount will be used for
      purposes of calculating the Reinvestment Rate.

SECTION 5. For the sole benefit of the Holders of the Senior Notes and Senior
Debentures, Section 106 shall be amended by deleting clause (1) thereof and
replacing such clause (1) in its entirety with the following:

            (1)   such notice shall be sufficiently given to Holders of
      Registered Securities (unless otherwise herein expressly provided) (a) if
      in writing and mailed, first-class postage prepaid, to each such Holder
      affected by such event, at the address of such Holder as it appears in the
      Security Register, not earlier than the earliest date, and not later than
      the latest date, prescribed for the giving of such notice, and (b) for so
      long as a series of Registered Securities is listed on the Luxembourg
      Stock Exchange and it is required by rules of the Luxembourg Stock
      Exchange, publication shall have been made of such notice with regard to
      such series of Securities in the English language in an Authorized
      Newspaper.

SECTION 6. For the sole benefit of the holders of the Senior Notes and the
Senior Debentures, Section 305 is hereby deleted in its entirety and replaced by
the following Sections 305, 305A and 305B:

                  SECTION 305. REGISTRAR AND PAYING AGENT. The Company shall
      maintain an office or agency in the City of New York where Securities may
      be presented for registration of transfer or for exchange (the
      "REGISTRAR"), an office or agency in the City of New York where Securities
      may be presented for payment (the "PAYING AGENT") and an office or agency
      where notices and demands to or upon the Company in respect of the
      Securities and this Indenture may be served, which shall be in the Borough
      of Manhattan, The City of New York and, in the event any series of
      Securities is listed on the Luxembourg Stock Exchange, in Luxembourg with
      respect to such series. The Company shall cause the Registrar to keep a
      register of the Securities and of their transfer and exchange (the
      "SECURITY REGISTER"). The Security Register shall be in written form or
      any other form capable of being converted into written form within a
      reasonable time. The Company may have one or more co-Registrars and one or
      more additional Paying Agents.

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                  The Company shall enter into an appropriate agency agreement
      with any Agent not a party to this Indenture. The agreement shall
      implement the provisions of this Indenture that relate to such Agent. The
      Company shall give prompt written notice to the Trustee of the name and
      address of any such Agent and any change in the address of such Agent. If
      the Company fails to maintain a Registrar, Paying Agent and/or agent for
      service of notices and demands, the Trustee shall act as such Registrar,
      Paying Agent and/or agent for service of notices and demands. The Company
      may remove any Agent upon written notice to such Agent and the Trustee;
      PROVIDED that no such removal shall become effective until (i) the
      acceptance of an appointment by a successor Agent to such Agent as
      evidenced by an appropriate agency agreement entered into by the Company
      and such successor Agent and delivered to the Trustee or (ii) notification
      to the Trustee that the Trustee shall serve as such Agent until the
      appointment of a successor Agent in accordance with clause (i) of this
      proviso. The Company, any Subsidiary of the Company, or any Affiliate of
      any of them may act as Paying Agent, Registrar or co-Registrar, and/or
      agent for service of notice and demands.

                  The Company initially appoints the Trustee as Registrar,
      Paying Agent, Authenticating Agent and agent for service of notice and
      demands and Kredietbank S.A. Luxembourg as Paying Agent and agent for
      service of notice and demands in Luxembourg, at Kredietbank S.A.
      Luxembourg, 43, Boulevard Royal, L-2955 Luxembourg; Attention: Corporate
      Trust Department. The Trustee shall preserve in as current a form as is
      reasonably practicable the most recent list available to it of the names
      and addresses of Holders and shall otherwise comply with TIA ss. 312(a).
      If the Trustee is not the Registrar, the Company shall furnish to the
      Trustee as of each Regular Record Date and at such other times as the
      Trustee may reasonably request the names and addresses of Holders as they
      appear in the Security Register, including the aggregate principal amount
      of Securities held by each Holder.

                  SECTION 305A. TRANSFER AND EXCHANGE. A Holder may transfer a
      Security only by written application to the Registrar stating the name of
      the proposed transferee and otherwise complying with the terms of this
      Indenture. No such transfer shall be effected until, and such transferee
      shall succeed to the rights of a Holder only upon, final acceptance and
      registration of the transfer by the Registrar in the Security Register.
      Prior to the registration of any transfer by a Holder as provided herein,
      the Company, the Trustee, and any agent of the Company shall treat the
      person in whose name the Security is registered as the owner thereof for
      all purposes whether or not the Security shall be overdue, and neither the
      Company, the Trustee, nor any such agent shall be affected by notice to
      the contrary. Furthermore, any Holder of a global Security shall, by
      acceptance of such global Security, agree that transfers of beneficial
      interests in such global Securities may be effected only through a book
      entry system maintained by the Holder of such global Security (or its
      agent) and that ownership of a beneficial

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      interest in the Security shall be required to be reflected in a book
      entry. When Securities are presented to the Registrar or a co-Registrar
      with a request to register the transfer or to exchange them for an equal
      principal amount of Securities of other authorized denominations, the
      Registrar shall register the transfer or make the exchange as requested if
      its requirements for such transactions are met (including that such
      Securities are duly endorsed or accompanied by a written instrument of
      transfer in form satisfactory to the Trustee and Registrar duly executed
      by the Holder thereof or by an attorney who is authorized in writing to
      act on behalf of the Holder). To permit registrations of transfers and
      exchanges, the Company shall execute and the Trustee shall authenticate
      Securities at the Registrar's request. No service charge shall be made for
      any registration of transfer or exchange or redemption of the Securities,
      but the Company may require payment of a sum sufficient to cover any
      transfer tax or similar governmental charge payable in connection
      therewith.

                  The Registrar shall not be required (i) to issue, register the
      transfer of or exchange any Security during a period beginning at the
      opening of business 15 days before the day of the mailing of a notice of
      redemption of Securities selected for redemption under Article Eleven and
      ending at the close of business on the day of such mailing, or (ii) to
      register the transfer of or exchange any Security so selected for
      redemption in whole or in part, except the unredeemed portion of any
      Security being redeemed in part.

                  SECTION 305B. BOOK-ENTRY PROVISIONS FOR GLOBAL SECURITIES. (a)
      Each global Security initially shall (i) be registered in the name of the
      Depositary for such global Security or the nominee of such Depositary, and
      (ii) be delivered, as applicable, either to the Trustee, as U.S.
      Depositary, or to the Common Depositary.

                  Members of, or Participants in, the Depositary ("AGENT
      MEMBERS") shall have no rights under this Indenture with respect to any
      global Security held on their behalf by the Depositary, or the Trustee as
      its custodian or the Common Depositary, as applicable, or under such
      global Security, and the Depositary may be treated by the Company, the
      Trustee and any agent of the Company or the Trustee as the absolute owner
      of such global Security for all purposes whatsoever. Notwithstanding the
      foregoing, nothing herein shall prevent the Company, the Trustee or any
      agent of the Company or the Trustee from giving effect to any written
      certification, proxy or other authorization furnished by the Depositary or
      impair, as between the Depositary and its Agent Members, the operation of
      customary practices governing the exercise of the rights of a holder of
      any Security.

                  (b)  Transfers of a global Security shall be limited to
      transfers of such global Security in whole, but not in part, to the
      Depositary, its successors or their respective nominees. Transfers of
      interests in one global Security to

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      parties who will hold the interests through the same global Security will
      be effected in the ordinary way in accordance with the respective rules
      and operating procedures of the applicable Depositaries. The provisions of
      the "Operating Procedures of the Euroclear System" and "Terms and
      Conditions Governing Use of Euroclear" of Euro-clear and the "General
      Terms and Conditions of Clearstream" and "Customer Handbook" of Cedel S.A.
      shall be applicable to interests in the global Securities that are held by
      Agent Members through Euro-clear and Cedel S.A.

                  (c)  Any beneficial interest in one of the global Securities
      that is transferred to a person who takes delivery in the form of an
      interest in another global Security will, upon transfer, cease to be an
      interest in such global Security and become an interest in such other
      global Security and, accordingly, will thereafter be subject to all
      transfer restrictions, if any, and other procedures applicable to
      beneficial interests in such other global Security for so long as it
      remains such an interest.

                  (d)  The registered holder of a global Security may grant
      proxies and otherwise authorize any person, including Agent Members and
      persons that may hold interests through Agent Members, to take any action
      which a Holder is entitled to take under this Indenture or the Securities.

                  (e)  The Securities are initially solely issuable as global
      Securities. Registered Securities shall only be transferred to all
      beneficial owners in exchange for their beneficial interests in a global
      Security, if (i) the Depositary with respect to such global Securities
      notifies the Company that it is unwilling or unable to continue as
      Depositary for such global Security, as the case may be, and a successor
      Depositary is not appointed by the Company within 90 days of such notice
      or (ii) an Event of Default has occurred and is continuing and the
      Registrar has received a request to the foregoing effect from the
      Depositary or the Trustee.

                  (f)  All Securities issued upon any transfer or exchange of
      Securities shall be valid, legally enforceable obligations of the Company,
      evidencing the same debt, and entitled to the same benefits under this
      Indenture, as the Securities surrendered upon such transfer or exchange.

SECTION 7. For the sole benefit of the Holders of the Senior Notes and Senior
Debentures, Section 307 shall be amended by deleting the first paragraph thereof
and replacing such paragraph in its entirety with the following:

                  (a)  Interest on any Registered Security which is payable, and
      is punctually paid or duly provided for, on any Interest Payment Date
      shall be paid, in immediately available funds, to the Person in whose name
      that Security (or one or more Predecessor Securities) is registered at the
      close of business on the Regular Record Date for such interest at the
      office or agency of the Company maintained for such purpose

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                                       8


      pursuant to Section 1002; PROVIDED, HOWEVER, that each installment of
      interest on any Registered Security may at the Company's option be paid in
      immediately available funds by transfer to an account maintained by the
      payee located in the United States.

SECTION 8. For the sole benefit of the Holders of the Senior Notes and Senior
Debentures, Section 501 shall be amended by deleting in subparagraphs (4) and
(5) thereunder, the references to the amount $100 million, and replacing such
references with the amount $250 million.

SECTION 9. For the sole benefit of the Holders of the Senior Notes and Senior
Debentures, the first paragraph of Section 1007 of the Indenture is hereby
deleted and replaced by the following paragraphs:

                  The Company shall, subject to the exceptions and limitations
      set forth below, pay as additional interest on the Securities of any
      series, such additional amounts (the "ADDITIONAL AMOUNTS") as are
      necessary in order that the net payment to be made by the Company or by a
      Paying Agent on behalf of the Company, of the principal of and interest on
      a series of Securities to a Holder who is a non-United States Person,
      after deduction for any present or future tax, assessment or other
      governmental charge of the United States or a political subdivision or
      taxing authority thereof or therein, imposed by withholding with respect
      to such payment, will not be less than the amount provided in the
      Securities to be then due and payable; PROVIDED, HOWEVER, that the
      foregoing obligation to pay Additional Amounts shall not apply:

            (1)   to any tax, assessment or other governmental charge that is
      imposed or withheld solely by reason of the Holder, or a fiduciary,
      settlor, beneficiary, member or shareholder of the Holder, if the Holder
      is an estate, trust, partnership or corporation, or a Person holding a
      power over an estate or trust administered by a fiduciary Holder, being
      considered as:

                  (a)  being or having been present or engaged in a trade or
            business in the United States or having had a permanent
            establishment in the United States;

                  (b)  having a current or former relationship with the United
            States, including a relationship as a citizen or resident thereof;

                  (c)  being or having been a foreign or domestic personal
            holding company, a passive foreign investment company or a
            controlled foreign corporation with respect to the United States or
            a corporation that has accumulated earnings to avoid United States
            federal income tax;

                  (d)  being or having a "10-percent shareholder" of the
            Company, as defined in Section 871(h)(3) of the United States
            Internal Revenue Code or any successor provision thereof; or

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                  (e)  being a bank receiving payments on an extension of credit
            made pursuant to a loan agreement entered into in the ordinary
            course of its trade or business;

            (2)   to any Holder that is not the sole beneficial owner of a
      Security, or a portion thereof, or that is a fiduciary or partnership, but
      only to the extent that a beneficiary or settlor with respect to the
      fiduciary, a beneficial owner or member of the partnership would not have
      been entitled to the payment of any Additional Amount had the beneficiary,
      settlor, beneficial owner or member received directly its beneficial or
      distributive share of the payment;

            (3)   to any tax, assessment or other governmental charge that is
      imposed or withheld by reason of the failure of the Holder or any other
      person to comply with certification, identification or information
      reporting requirements concerning the nationality, residence, identity or
      connection with the United States, or otherwise with respect to the
      status, of the Holder or beneficial owner of such Security (or any
      beneficiary, settlor, beneficial owner or member thereof), if compliance
      is required by statute, by regulation of the United States Treasury
      Department or by an applicable income tax treaty to which the United
      States is a party, or by any official interpretation or ruling promulgated
      pursuant to any of the foregoing, as a precondition to exemption from such
      tax, assessment or other governmental charge;

            (4)   to any tax, assessment or other governmental charge that is
      imposed otherwise than by withholding by the Company or by a Paying Agent
      on its behalf from the payment;

            (5)   to any tax, assessment or other governmental charge that is
      imposed or withheld solely by reason of a change in law, regulation, or
      administrative or judicial interpretation that becomes effective more than
      15 days after the payment becomes due or is duly provided for, whichever
      occurs later;

            (6)   to any estate, inheritance, gift, sales, excise, transfer,
      wealth or personal property tax or similar tax, assessment or other
      governmental charge;

            (7)   to any tax, assessment or other governmental charge required
      to be withheld by any Paying Agent from any payment of principal of or
      interest on any Security, if such payment can be made without such
      withholding by any other Paying Agent on behalf of the Company; or

            (8)   in the case of any combination of items (1), (2), (3),
      (4), (5), (6) and (7).

                  The Securities are subject in all cases to any tax, fiscal or
      other law or regulation or administrative or judicial interpretation
      applicable thereto. Except as otherwise provided for in this Section 1007
      and Section 1101, the Company shall not be required to make any payments
      with respect to any tax,

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                                       10


      assessment or other governmental charge imposed by any government or a
      political subdivision or taxing authority thereof or therein.

                  Whenever in this Indenture there is mentioned, in any context,
      the payment of the principal of (or premium, if any, on) or interest on,
      or in respect of, any Security of a series or payment of any related
      coupon or the net proceeds received on the sale or exchange of a Security
      of a series, such mention shall be deemed to include mention of the
      payment of Additional Amounts provided for by the terms of such series
      established pursuant to Section 301 to the extent that, in such context,
      Additional Amounts are, were or would be payable in respect thereof
      pursuant to such terms and express mention of the payment of Additional
      Amounts (if applicable) in any provisions hereof shall not be construed as
      excluding Additional Amounts in those provisions hereof where such express
      mention is not made.

SECTION 10. For the sole benefit of the Holders of the Senior Notes and the
Senior Debentures, Section 1010 of the Indenture is hereby deleted in its
entirety and replaced by the following Section 1010:

                  Section 1010. LIMITATIONS ON LIENS.

                  The Company shall not create, assume or suffer to exist any
      Lien on any of its property or assets, without securing the Securities of
      any applicable series equally and ratably with (or prior to) such secured
      Indebtedness; PROVIDED, HOWEVER, that the foregoing shall apply only to
      Liens which in the aggregate exceed 15% of the Company's total
      consolidated assets as of the end of the Company's most recent accounting
      period preceding the creation or assumption of any such Lien (reduced by
      any Attributable Debt with respect to any Sale and Leaseback Transaction
      permitted under Section 1011 below). This restriction will not apply to
      Capitalized Leases or to Indebtedness secured by (a) Liens existing on
      August 1, 2000 and Liens on property of, or Indebtedness of, any Person at
      the time such Person becomes a Subsidiary (whether by acquisition or
      otherwise, including through merger or consolidation), (b) Liens in favor
      of the Company or a Subsidiary of the Company, (c) Liens existing at the
      time of acquisition of the assets secured thereby (including acquisition
      through merger or consolidation) and purchase money Liens, and (d) any
      extension, renewal or refunding of any Lien referred to in the foregoing
      clauses (a) through (c), inclusive.

SECTION 11. For the sole benefit of the Holders of the Senior Notes and Senior
Debentures, Section 1101 of the Indenture is hereby deleted in its entirety and
replaced by the following Section 1101:

                  Section 1101.  OPTIONAL REDEMPTION.

                  In the event that as a result of any change in, or amendments
      to, any laws (or any regulations or rulings promulgated thereunder) of the
      United

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                                       11


      States (or any political subdivision or taxing authority thereof or
      therein) or any change in, or amendments to, an official position
      regarding the application of such laws, regulations or rulings, which
      change or amendment is announced or becomes effective thereunder after
      July 25, 2000, the Company has become or, based upon a written opinion of
      independent counsel selected by the Company, will become obligated to pay,
      with respect to a series of Securities, any Additional Amounts, the
      Company may redeem, in accordance with this Article Eleven, all, but not
      less than all, the Securities of such series at any time at 100% of the
      principal amount thereof, together with accrued interest thereon, if any,
      to the Redemption Date (subject to the rights of holders of record on the
      relevant Regular Record Date that is prior to the Redemption Date to
      receive interest on the relevant Interest Payment Date).

                  In addition, the Securities of any given series will be
      redeemable, in accordance with this Article Eleven, at any time, at the
      option of the Company, in whole or from time to time in part, upon not
      less than 30 nor more than 60 days' prior notice, on any date prior to
      their maturity at a Redemption Price equal to the sum of 100% of the
      principal amount thereof and the Make-Whole Amount and any accrued and
      unpaid interest, to the Redemption Date (subject to the rights of holders
      of record on the relevant Regular Record Date that is on or prior to the
      Redemption Date to receive interest due on the relevant Interest Payment
      Date). The Make-Whole Amount with respect to such a redemption shall be
      calculated by an independent investment banking institution of national
      standing appointed by the Company. If the Company fails to so appoint an
      independent investment banking institution at least 45 Business Days prior
      to the Redemption Date, or if such institution is unwilling or unable to
      calculate the Make-Whole Amount, the calculation of such Make-Whole Amount
      shall be made by an independent investment banking institution of national
      standing appointed by the Trustee. If, for purposes of calculating the
      Make-Whole Amount, the Reinvestment Rate shall not be available as set
      forth in the definition thereof, the Reinvestment Rate shall be calculated
      by interpolation or extrapolation of comparable rates selected by the
      independent investment banking institution.

SECTION 12. THE LAWS OF THE STATE OF NEW YORK SHALL GOVERN THIS FOURTH
SUPPLEMENTAL INDENTURE.

SECTION 13. This Fourth Supplemental Indenture may be executed in any number of
counterparts, each of which when so executed shall be deemed to be an original,
but such counterparts shall together constitute but one and the same instrument.

SECTION 14. Except as herein amended with respect to the Senior Notes and Senior
Securities, all applicable terms, conditions and provisions of the Indenture, as
supplemented, shall continue in full force and effect and shall remain binding
and enforceable in accordance with their respective terms.


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                                       12



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<PAGE>
                                       13


            IN WITNESS WHEREOF, the parties have caused this Fourth Supplemental
Indenture to be duly executed and attested, all as of the day and year first
written above.


                                       VIACOM INC.


                                       By:  /s/ Michael D. Fricklas
                                            -------------------------------
                                            Name:  Michael D. Fricklas
                                            Title: Executive Vice President,
                                                   General Counsel and
                                                   Secretary


ATTEST

By:  /s/ Laura Franco
     ---------------------------------
     Name:  Laura Franco
     Title: Assistant Secretary


                                       VIACOM INTERNATIONAL INC.


                                       By:  /s/  Michael D. Fricklas
                                            ----------------------------------
                                            Name:  Michael D. Fricklas
                                            Title: Executive Vice President,
                                                   General Counsel and
                                                   Secretary


ATTEST

By:  /s/ Laura Franco
     ---------------------------------
     Name:  Laura Franco
     Title: Assistant Secretary


                                       CITIBANK, N.A.

                                       By:  /s/ Pat DeFelice
                                            ----------------------------------
                                            Name:  Pat DeFelice
                                            Title: Vice President


ATTEST

By:  /s/ Wafaa Orfy
     ---------------------------------
     Name:  Wafaa Orfy
     Title: Assistant Vice President