Document:

<PAGE>

                                                                   EXHIBIT 10.16
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                              THIRD SUPPLEMENT TO

                             INTER-GROUP AGREEMENT

                               between and among

                                  AT&T CORP.,

                               on the one hand,

                                      and

                          LIBERTY MEDIA CORPORATION,

                            LIBERTY MEDIA GROUP LLC

                     and each Covered Entity listed on the

                            signature pages hereof,

                              on the other hand,

                         dated as of October 20, 1999

--------------------------------------------------------------------------------
<PAGE>

                              THIRD SUPPLEMENT TO

                             INTER-GROUP AGREEMENT

          Agreement dated as of October 20, 1999 between AT&T Corp., a New York
corporation ("AT&T"), for itself and on behalf of the members of the Common
              ----
Stock Group, on the one hand, and Liberty Media Corporation, a Delaware
corporation ("LMC"), Liberty Media Group LLC, a Delaware limited liability
              ---
company, and for so long as such Covered Entity remains a Covered Entity under
the applicable provisions of the AT&T Charter Amendment, each Covered Entity
listed on the signature pages hereof (collectively, the "Liberty Media
                                                         -------------
Parties"), for themselves and, in the case of LMC, on behalf of the other
-------
members of the Liberty Media Group, on the other hand. Capitalized terms used
herein without definition have the meanings ascribed to such terms in (i) the
Ascent Merger Agreement (as hereinafter defined), or (ii) if not otherwise
defined herein or in the Ascent Merger Agreement, the Inter-Group Agreement (as
hereinafter defined).

          WHEREAS, AT&T and the Liberty Media Parties are parties to that
certain Inter-Group Agreement, dated as of March 9, 1999 (the "Inter-Group
                                                               -----------
Agreement"), as supplemented and modified by the First Supplement to Inter-Group
---------
Agreement, dated as of May 28, 1999 (the "First Supplement"), and the Second
                                          ----------------
Supplement to Inter-Group Agreement, dated as of September 24, 1999 (the "Second
                                                                          ------
Supplement"), which establishes certain terms and conditions concerning the
----------
responsibilities and obligations of each Group to the other as well as certain
additional provisions concerning the Groups' relationships with each other;

          WHEREAS, AT&T, Ranger Acquisition Corp., a Delaware corporation

("Merger Sub"), LMC and Ascent Entertainment Group, Inc., a Delaware corporation
  ----------
("Ascent"), intend to
  ------
<PAGE>

enter into an Agreement and Plan of Merger, dated as of October 20, 1999 (the
"Ascent Merger Agreement"), pursuant to which, among other things, subject to
 -----------------------
the terms and conditions contained in the Ascent Merger Agreement, Merger Sub
will be merged with and into Ascent and Ascent, as the Surviving Entity of such
merger, will initially become a direct wholly owned subsidiary of AT&T (the
"Ascent Merger");
 -------------

          WHEREAS, in connection herewith AT&T, LMC and certain of their
respective affiliates are also entering into a Third Amendment to Tax Sharing
Agreement (the "Third Tax Sharing Amendment"), dated as of the date hereof,
                ---------------------------
which amends that certain Tax Sharing Agreement, dated as of March 9, 1999, as
amended, (as so amended, the "Tax Sharing Agreement"), to which AT&T, LMC and
                              ---------------------
certain of their respective affiliates are parties;

          WHEREAS, as a condition to the willingness of each of AT&T and LMC to
enter into the Ascent Merger Agreement, AT&T and the Liberty Media Parties have
determined to enter into this Agreement and the Third Tax Sharing Amendment;

          WHEREAS, AT&T and the Liberty Media Parties desire to supplement and
modify the Inter-Group Agreement in the manner set forth in this Agreement and
to establish in this Agreement certain terms and conditions concerning the
responsibilities and obligations of each Group to the other as well as certain
additional provisions concerning each Group's relationship with each other as
the same may relate to the Ascent Merger, and certain related transactions;

          WHEREAS, in connection herewith, the Capital Stock Committee has
adopted the resolutions attached as Exhibit A.

                                       2
<PAGE>

          NOW, THEREFORE, in consideration of the foregoing and the mutual
covenants and agreements contained herein, and intending to be legally bound
hereby, AT&T and the Liberty Media Parties hereby agree as follows:

                                   ARTICLE I

                   OBLIGATIONS RELATING TO THE ASCENT MERGER

          SECTION 1.1.  Post-Closing Contribution. As soon as reasonably
                        -------------------------
practicable following the effectiveness of the Ascent Merger, each of LMC and
AT&T shall use all reasonable efforts to engage in the Post-Merger Restructuring
Transactions. Each of AT&T and LMC shall take, and AT&T shall cause each of its
Subsidiaries to take, any and all required actions (whether as a stockholder (or
other interest holder) or through its respective representatives on the board of
directors (or comparable governing body) of the applicable entity), to give
effect to the previous sentence.

          SECTION 1.2.  Certain Liabilities Relating to the Ascent Merger.
                        -------------------------------------------------

          (a)   Subject to the last sentence of this subsection (a), as between
the Common Stock Group and the Liberty Media Group, all of the following
Liabilities shall be Liabilities of the Liberty Media Group:

          (i)   all Liabilities of Ascent and any of its subsidiaries and
                affiliates (and, in each case, any predecessor or successor
                thereto);

          (ii)  all Liabilities arising out of or related to the Ascent Merger
                Agreement and the transactions contemplated thereby, including
                (A) all Liabilities arising out of or relating to any breach of
                the Ascent Merger Agreement by LMC or by Ascent (whether such
                breach is of a representation, warranty, agreement or

                                       3
<PAGE>

                obligation made to AT&T, LMC or otherwise, and without reference
                to any qualification of any such representation, warranty,
                agreement or obligation by reference to materiality, the Company
                SEC Reports or the Company Disclosure Schedule, and without
                reference to any limitation or survival of such representation,
                warranty, covenant or obligation set forth in the Ascent Merger
                Agreement and without regard to Section 10.10 of the Ascent
                Merger Agreement), (B) any Liabilities to third parties
                contemplated by Section 4.10 of the Ascent Merger Agreement, (C)
                all Liabilities required to carry out the provisions of Sections
                2.1 and 2.4 of the Ascent Merger Agreement, (D) all Liabilities
                under Sections 4.22 and 7.6 of the Ascent Merger Agreement, and
                (E) all Liabilities arising out of or related to lawsuits
                brought, proposed or threatened by any third party (other than
                Ascent) against AT&T or Merger Sub (a "Third Party Lawsuit")
                                                       -------------------
                arising from the execution, delivery or performance of the
                Ascent Merger Agreement or this Agreement by AT&T or Merger Sub,
                and (F) relieving AT&T and any member of the Common Stock Group
                of any Liability or obligation that binds or is enforceable
                against AT&T or such member of the Common Stock Group as a
                result of any agreement between Ascent and its Subsidiaries, on
                the one hand, and any other person (other than an agreement
                executed by AT&T or a member of the Common Stock Group), on the
                other hand;

          (iii) all Liabilities arising out of the consummation of the Ascent
                Merger or any of the transactions contemplated thereby,
                including the Post-Merger

                                       4
<PAGE>

                Restructuring Transactions and any other transaction
                contemplated by this Agreement or the Ascent Merger Agreement,
                notwithstanding the failure to receive any consent, approval or
                authorization, or any failure to make any filing or
                notification, from or to any Person that may be required in
                connection therewith;

          (iv)  all Liabilities arising out of the consummation of the Post-
                Merger Restructuring Transactions;

          (v)   all Liabilities relating to current or former employees,
                officers, directors, consultants and other agents of Ascent and
                any of its current or former Subsidiaries and Affiliates,
                including, all Liabilities contemplated by Sections 3.6 and 3.8
                of the Ascent Merger Agreement and all Company Stock Options and
                Company SARs assumed by AT&T and other Stock Incentives created
                pursuant to the Ascent Merger Agreement; and

          (vi)  except to the extent resulting from any breach of this Agreement
                or the Ascent Merger Agreement by AT&T that does not arise out
                of or relate to actions taken by AT&T at the request of LMC (as
                contemplated by Section 1.2(d) hereof or otherwise in writing),
                all Liabilities arising out of the execution, delivery or
                performance of this Agreement.

Notwithstanding the foregoing, as between the Common Stock Group and the Liberty
Media Group, the Common Stock Group shall be responsible for (i) any Liabilities
(other than any Liability arising from or relating to an Ascent Tax Item (as
defined in the Third Tax Sharing Amendment) to the extent arising out of or
relating to any breach by AT&T or Merger Sub of the Ascent Merger

                                       5
<PAGE>

Agreement or this Agreement that do not arise out of or relate to actions taken
by AT&T at the request of LMC (as contemplated by Section 1.2(d) hereof or
otherwise in writing), whether incurred by a member of the Common Stock Group or
by a member of the Liberty Media Group, and (ii) any Ascent Tax Item (as defined
in the Third Tax Sharing Amendment), to the extent set forth in Section
3(d)(xii) of the Tax Sharing Agreement.

          (b) The Liberty Media Group shall be responsible for, and shall
reimburse the Common Stock Group for, any and all reasonable costs, fees and
expenses incurred by AT&T or any member of the Common Stock Group in connection
with:  (i) the negotiation, review, execution and delivery of the Ascent Merger
Agreement, this Supplement and the Third Tax Sharing Amendment; (ii) the
consummation of the transactions contemplated by the Ascent Merger Agreement and
this Agreement; (iii) the preparation, review and filing of the Registration
Statement (and the Proxy Statement that will be a part thereof); (iv) the
consummation of the Post-Merger Restructuring Transactions; (v) making any
filings with any governmental entity and otherwise assisting in obtaining
approvals, consents and clearances required in connection with the transactions
contemplated by the Ascent Merger Agreement and this Supplement, if any; and (vi
all other fees and expenses related thereto, including in each case any internal
costs, fees and expenses (which shall be determined in any reasonable manner
developed by AT&T for tracking such internal costs, fees and expenses), and in
each case such costs, fees and expenses to be reimbursed promptly upon receipt
of a statement therefor.

          (c) LMC shall indemnify and hold harmless AT&T and each member of the
Common Stock Group from and against, and pay and reimburse AT&T and each member
of the Common Stock Group for, any and all Liabilities (including reasonable
attorneys' fees and expenses)

                                       6
<PAGE>

for which the Liberty Media Group is responsible pursuant to this Section 1.2 in
each case in accordance with Section 1.4(c) of the Inter-Group Agreement.

          (d) Notwithstanding any provision of the Ascent Merger Agreement, AT&T
and Merger Sub shall not waive any condition to their respective obligations to
consummate the Ascent Merger set forth in Article VIII of the Ascent Merger
Agreement (the "Parent Closing Conditions") without LMC's prior written consent;
                -------------------------
provided, however, that consummation of the Ascent Merger by Parent based on the
--------  -------
reasonable belief that all of the Parent Closing Conditions have been satisfied
shall not be deemed a breach of this Agreement unless (and to the extent) AT&T
receives a written request referred to in the next sentence at least 24 hours
prior to the then scheduled Closing of the Ascent Merger.  To the extent so
requested in writing by LMC, each of AT&T and Merger Sub shall (i) assert that
one or more of the Parent Closing Conditions have not been satisfied and decline
to consummate the transactions contemplated by the Ascent Merger Agreement or
(ii) to the extent lawful, waive one or more of the Parent Closing Conditions
(other than those set forth in Section 8.1 of the Ascent Merger Agreement). The
Liberty Media Group agrees to indemnify AT&T for any Liabilities incurred by
AT&T or any member of the Common Stock Group as a result of the taking of an
action by AT&T or Merger Sub at the request of LMC as contemplated by the
previous sentence (but without regard to whether such action constitutes a
breach of the Ascent Merger Agreement).  Notwithstanding clause (ii) of the
second sentence of this paragraph, AT&T shall not be obligated to waive any
Parent Closing Condition and consummate the Ascent Merger if AT&T, in its
reasonable judgment, determines that the foregoing agreement of LMC to indemnify
AT&T from and against any Liability incurred as a result of such action is not
adequate to fully protect AT&T and the Common Stock Group from and against any
Liability that could reasonably be

                                       7
<PAGE>

expected to result from such waiver and the consummation of the Ascent Merger in
connection therewith (it being understood that no such indemnification would be
adequate if such Liability relates to, without limitation, any injunction
applicable to AT&T or any of its Subsidiaries, any restriction on the operation
of any business or assets of AT&T or any of its Subsidiaries or the relationship
of AT&T or any of its Subsidiaries with any governmental entity that has
jurisdiction over AT&T or such Subsidiary or any of their assets or businesses).
Except as set forth in this paragraph, nothing herein shall prohibit AT&T or
Merger Sub from exercising any of its rights under the Ascent Merger Agreement.
AT&T shall reasonably consider any amendments to, or any waiver of any other
provision of, the Ascent Merger Agreement proposed by LMC. If so requested by
AT&T, in connection with the closing of the Ascent Merger, LMC shall acknowledge
in writing to AT&T that the consummation of the Ascent Merger would not be
inconsistent with AT&T's obligations under this paragraph, and the failure of
AT&T to consummate the Ascent Merger in the absence of such acknowledgment shall
be deemed, for all purposes of this Agreement, not to be a violation of this
Agreement or the Ascent Merger Agreement.

          (e) All employees, officers, directors, consultants and other agents
of Ascent and its Subsidiaries shall be deemed to be employees, officers,
directors, consultants and other agents of the Liberty Media Group, and all
Stock Incentives (including without limitation any obligations pursuant to
Section 2.3(b) of the Ascent Merger Agreement) that are created as a result of
the Ascent Merger shall be the responsibility of the Liberty Media Group.

          (f) For purposes of Section 1.16 of the Inter-Group Agreement, the
Company Stock Options and Company SARs to be assumed at the effective time of
the Ascent Merger, the obligation to issue Class A AT&T Liberty Tracking Shares
to Ascent shareholders pursuant to the

                                       8
<PAGE>

Ascent Merger Agreement, and any other obligation to issue or sell any Class A
AT&T Liberty Tracking Shares pursuant to or in connection with the Ascent
Merger, the Ascent Merger Agreement, and any agreement referred to in the Ascent
Merger Agreement or executed in connection therewith shall be deemed to be
Tracking Stock Obligations incurred with a Liberty Approval. The issuance of
Class A AT&T Liberty Tracking Shares pursuant to the Ascent Merger and any
transaction contemplated by the Ascent Merger Agreement shall be deemed to have
occurred with a Liberty Approval for purposes of Section 1.11(a) of the Inter-
Group Agreement and shall not be a breach of Section 1.11(b) or (c) of the
Inter-Group Agreement. Subject to Section 1.16 of the Inter-Group Agreement, any
action taken by AT&T to implement the terms of this paragraph (f) or Sections
2.3(b) and 3.6 of the Ascent Merger Agreement shall not constitute a breach of
any provision of the Inter-Group Agreement.

          (g) At all times from the date of its formation until the Effective
Time of the Ascent Merger, Merger Sub shall be a direct, wholly owned Subsidiary
of AT&T.

          SECTION 1.3  Allocation of Proceeds. For purposes of Section 1.6(b) of
                       ----------------------
the Inter-Group Agreement and the definition of the term "Liberty Media Group"
in the AT&T Charter Amendment, the net proceeds of the issuance of the Class A
AT&T Liberty Tracking Shares to be issued in the Merger shall consist of the
entire interest of AT&T in the Surviving Entity and each of its Subsidiaries.
Following the Effective Time, the Surviving Entity shall be a member of the
Liberty Media Group.

          In furtherance of the foregoing, any contract right or other similar
right associated with any asset that is attributed to a Group in accordance with
the preceding paragraph shall be the right of the Group to which such related
asset is so attributed.

                                       9
<PAGE>

          SECTION 1.4.   Certain Obligations.
                         -------------------

          (a) AT&T agrees to reasonably consider any actions that it may be
requested by LMC to take in accordance with consummation of the Ascent Merger.

          (b) LMC represents and warrants that true and complete copies of all
documents referred to in the Ascent Merger Agreement that have been delivered to
LMC by or on behalf of the Company on or prior to the date hereof have been
delivered to AT&T (or its counsel) on or prior to the date hereof and agrees
that true and complete copies of all such documents delivered to LMC from and
after the date hereof until consummation of the transactions contemplated hereby
will be delivered promptly to AT&T (or its counsel).

          (c) To the fullest extent possible, LMC will perform all obligations
of AT&T under Section 3.4 of the Ascent Merger Agreement, such as preparation of
documents and filings, and seeking necessary consents.  In connection with the
foregoing, LMC will not make any filing or take any action in the name of or on
behalf of AT&T without receiving the prior written consent of AT&T.

          (d) AT&T acknowledges that its covenants in this Agreement and the
Ascent Merger Agreement are subject to the fiduciary duties of its board of
directors to all of its stock-holders, including the holders of AT&T Common
Shares and the holders of AT&T Liberty Tracking Shares, in each case in
accordance with the Parent Charter and the Policy Statement regarding Liberty
Media Group Tracking Stock Matters.

                                      10
<PAGE>

                                  ARTICLE II
                                  DEFINITIONS

          SECTION 2.1.   Certain Definitional Provisions.  The language used in
                         -------------------------------
this Agreement shall be deemed to be the language chosen by the parties to
express their mutual intent, and no rule of strict construction shall be applied
against any party. Any references to any statute or law shall also refer to all
rules and regulations promulgated thereunder, unless the context requires
otherwise. Whenever the context may require, any pronoun shall include the
corresponding masculine, feminine and neuter forms. The words "include,"
"includes," and "including" shall be deemed to be followed by the phrase
"without limitation". The words "hereof," "herein" and "hereunder" and words of
similar import when used in this Agreement shall refer to this Agreement as a
whole and not to any particular provision of this Agreement, and Article and
Section references are to this Agreement unless otherwise specified. The
meanings given to terms defined herein shall be equally applicable to both the
singular and plural forms of such terms. Unless the context shall otherwise
require, any references to any agreement or other instrument or statute or
regulation are to it as amended and supplemented from time to time (and, in the
case of a statute or regulation, to any successor provisions). Any reference in
this Agreement to a "day" or number of "days" (without the explicit
qualification of "business") shall be interpreted as a reference to a calendar
day or number of calendar days. If any action or notice is to be taken or given
on or by a particular calendar day, and such calendar day is not a business day,
then such action or notice shall be deferred until, or may be taken or given on,
the next business day.

                                      11
<PAGE>

                                  ARTICLE III
                                 MISCELLANEOUS

          SECTION 3.1.   Notices. All notices, requests, demands or other
                         -------
communications required by or otherwise with respect to this Agreement shall be
in writing and shall be deemed to have been given to any party when delivered
personally (by courier service or otherwise), when delivered by telecopy and
confirmed by return telecopy, or upon the receipt after being mailed by first-
class mail, postage prepaid and return receipt requested in each case to the
applicable addresses set forth below:

     If to AT&T or any member of the Common Stock Group:

          AT&T Corp.
          295 North Maple Avenue
          Basking Ridge, New Jersey 07920
          Attention:     Vice President-Law
                         and Corporate Secretary
          Facsimile:     (908) 221-6618

     with a copy to:

          Wachtell, Lipton, Rosen & Katz
          51 West 52nd Street
          New York, New York 10019
          Attention:     Richard D. Katcher, Esq.
                         Steven A. Rosenblum, Esq.
          Facsimile:     (212) 403-2000

     If to LMC or any member of the Liberty Media Group:

          Liberty Media Corporation
          9197 South Peoria Street
          Englewood, Colorado 80112
          Attention:     Charles Y. Tanabe, Esq.
          Facsimile:     (720) 875-5382

                                      12
<PAGE>

     with a copy to:

          Baker & Botts, L.L.P.
          599 Lexington Avenue
          New York, New York 10022
          Attention:     Elizabeth M. Markowski, Esq.
                         Lee D. Charles, Esq.
          Facsimile:     (212) 705-5125

or such address as such party shall have designated by notice so given to each
other party.

          SECTION 3.2.   Amendments; No Waivers.  (a) This Agreement shall be
                         ----------------------
amended, changed, supplemented, waived or otherwise modified only by an
instrument in writing signed by each of AT&T and LMC (and following a Triggering
Event, Liberty Media Group LLC).

          (b) No failure or delay by any party in exercising any right, power or
privilege hereunder shall operate as a waiver thereof nor shall any single or
partial exercise thereof preclude any other or further exercise thereof or the
exercise of any other right, power or privilege.  The rights and remedies herein
provided shall be cumulative and not exclusive of any rights or remedies
provided by law.

          SECTION 3.3.   Successors and Assigns.  Neither this Agreement nor any
                         ----------------------
of the rights or obligations under this Agreement shall be assigned, in whole or
in part, by any party without the prior written consent of the other parties
hereto; provided, however, that the assignment of its rights and obligations
        --------  -------
under this Agreement by LMC or any Covered Entity to Liberty Media Group LLC in
connection with the transactions contemplated by the Contribution Agreement
shall not require the consent of AT&T.  Subject to the foregoing, the provisions
of this Agreement shall be binding upon and inure to the benefit of the parties
hereto and their respective successors and assigns.

                                      13
<PAGE>

          SECTION 3.4.   Governing Law; Consent to Jurisdiction.  This Agreement
                        --------------------------------------
and all disputes hereunder shall be governed by and construed and enforced in
accordance with the internal laws of the State of Delaware, without regard to
the principles of conflicts of laws. Each party hereto irrevocably and
unconditionally consents to submit to the exclusive jurisdiction of the United
States District Court for the District of Delaware or the Chancery Court of the
State of Delaware in any action, suit or proceeding arising in connection with
this Agreement, and agrees that any such action, suit or proceeding shall be
brought only in such court (and waives any objection based on forum non
                                                              ----- ---
coveniens or any other objection to venue therein); provided, however, that such
---------                                           --------  -------
consent to jurisdiction is solely for the purpose referred to in this Section
3.4 and shall not be deemed to be a general submission to the jurisdiction of
said courts or of the State of Delaware other than for such purpose.  AT&T and
LMC each hereby waive any right to a trial by jury in connection with any such
action, suit or proceeding.

          SECTION 3.5.   Counterparts; Effectiveness.  This Agreement may be
                         ---------------------------
executed in any number of counterparts, each of which shall be deemed to be an
original, but all of which together shall constitute one instrument. This
Agreement shall become effective when each party hereto shall have received
counterparts thereof signed by the other party hereto.

          SECTION 3.6.   Specific Performance.  Each of AT&T and LMC
                         --------------------
acknowledges and agrees that money damages are not an effective remedy for
violations of this Agreement and that any party may, in its sole discretion,
apply to a court of competent jurisdiction for specific performance or
injunctive or such other relief as such court may deem just and proper in order
to enforce this Agreement or prevent any violation hereof and, to the extent
permitted by applicable Law, each party waives any objection to the imposition
of such relief.

                                      14
<PAGE>

          SECTION 3.7.   Remedies Cumulative.  All rights, powers and remedies
                         -------------------
provided under this Agreement or otherwise available in respect hereof at law or
in equity shall be cumulative and not alternative, and the exercise or beginning
of the exercise of any thereof by any party shall not preclude the simultaneous
or later exercise of any other such right, power or remedy by such party.

          SECTION 3.8.   Termination.  This Agreement shall remain in full force
                         -----------
and effect until the earlier to occur of (i) such time as no Class A AT&T
Liberty Tracking Shares or Class B AT&T Liberty Tracking Shares are outstanding
and (ii) the termination of the Ascent Merger Agreement, at which time this
Agreement shall terminate and upon termination, no party shall have any
liability or further obligation to the other under this Agreement except that
the provisions of this Section 3.8 shall survive the termination of this
Agreement; provided, however, that such termination shall not relieve any party
hereto of any liability for any breach of this Agreement or the Ascent Merger
Agreement. No termination of this Agreement shall limit or otherwise affect the
rights or obligations of the parties to the Inter-Group Agreement.

          SECTION 3.9.   Severability.  In case any provision in this Agreement
                         ------------
shall be held invalid, illegal or unenforceable in a jurisdiction, such
provision shall be modified or deleted, as to the jurisdiction involved, only to
the extent necessary to render the same valid, legal and enforceable, and the
validity, legality and enforceability of the remaining provisions hereof shall
not in any way be affected or impaired thereby nor shall the validity, legality
or enforceability of such provision be affected thereby in any other
jurisdiction.

                                      15
<PAGE>

          SECTION 3.10.  Cooperation.  Each of AT&T and LMC covenants and agrees
                         -----------
with the other to use its reasonable best efforts to cause each member of the
Common Stock Group and each member of the Liberty Media Group, respectively, to
fulfill each of its respective obligations under this Agreement.
<PAGE>

          IN WITNESS WHEREOF, the parties hereto have caused this Agreement to
be duly executed as of the day and year first above written.

                              AT&T CORP.

                              By:  /s/ Daniel E. Somers
                                 ------------------------------------
                                  Name:    Daniel E. Somers
                                  Title:   Senior Executive Vice President and
                                           Chief Financial Officer

                              LIBERTY MEDIA CORPORATION

                              By:  /s/ Gary S. Howard
                                 ------------------------------------
                                    Name:  Gary S. Howard
                                    Title: Executive Vice President and Chief
                                           Operating Officer

                              LIBERTY MEDIA GROUP LLC

                              By:  /s/ Gary S. Howard
                                 ------------------------------------
                                    Name:  Gary S. Howard
                                    Title: Vice President
<PAGE>

                              Each of the following Covered Entities hereby
                              executes this Agreement as a member of the Liberty
                              Media Group to become a party to this Agreement
                              for so long as it remains a Covered Entity under
                              the applicable provisions of the AT&T Charter
                              Amendment:

                              LIBERTY SP, INC.

                              By:  /s/ Gary S. Howard
                                 ------------------------------------
                                    Name:  Gary S. Howard
                                    Title: Executive Vice President and Chief
                                           Operating Officer

                              LIBERTY AGI, INC.

                              By:  /s/ Gary S. Howard
                                 ------------------------------------
                                    Name:  Gary S. Howard
                                    Title: Executive Vice President and Chief
                                           Operating Officer

                              LMC INTERACTIVE, INC.

                              By:  /s/ Gary S. Howard
                                 ------------------------------------
                                    Name:  Gary S. Howard
                                    Title: Executive Vice President and Chief
                                           Operating Officer<PAGE>

                                                                   EXHIBIT 10.17

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

                             FOURTH SUPPLEMENT TO

                             INTER-GROUP AGREEMENT

                               between and among

                                  AT&T CORP.,

                               on the one hand,

                                      and

                          LIBERTY MEDIA CORPORATION,

                            LIBERTY MEDIA GROUP LLC

                     and each Covered Entity listed on the

                            signature pages hereof,

                              on the other hand,

                         dated as of December 6, 1999

--------------------------------------------------------------------------------
<PAGE>

                             FOURTH SUPPLEMENT TO

                             INTER-GROUP AGREEMENT

          Agreement dated as of December 6, 1999 (this "Agreement") between AT&T
Corp., a New York corporation ("AT&T"), for itself and on behalf of the members
                                ----
of the Common Stock Group, on the one hand, and Liberty Media Corporation, a
Delaware corporation ("LMC"), Liberty Media Group LLC, a Delaware limited
                       ---
liability company, and for so long as such Covered Entity remains a Covered
Entity under the applicable provisions of the AT&T Charter Amendment, each
Covered Entity listed on the signature pages hereof (collectively, the "Liberty
                                                                        -------
Media Parties"), for themselves and, in the case of LMC, on behalf of the other
-------------
members of the Liberty Media Group, on the other hand.

          WHEREAS, AT&T and the Liberty Media Parties are parties to that
certain Inter-Group Agreement, dated as of March 9, 1999 (the "Inter-Group
                                                               -----------
Agreement"), as supplemented and modified by (i) the First Supplement to Inter-
---------
Group Agreement, dated as of May 28, 1999, as such First Supplement has been
amended (the "First Supplement"), (ii) the Second Supplement to Inter-Group
              ----------------
Agreement, dated as of September 24, 1999 (the "Second Supplement") and (iii)
                                                -----------------
the Third Supplement to Inter-Group Agreement, dated as of October 20, 1999 (the
"Third Supplement"), which establishes certain terms and conditions concerning
 ----------------
the responsibilities and obligations of each Group to the other as well as
certain additional provisions concerning the Groups' relationships with each
other;
<PAGE>

          WHEREAS, AT&T, D-Group Merger Corp., a Delaware corporation ("Merger
                                                                        ------
Sub D"), LMC, and Four Media Company, a Delaware corporation ("Four Media"),
-----                                                          ----------
intend to enter into an Agreement and Plan of Merger, dated as of December 3,
1999 (the "Four Media Merger Agreement"), pursuant to which, among other things,
           ---------------------------
subject to the terms and conditions contained in the Four Media Merger
Agreement, Merger Sub D will be merged with and into Four Media and Four Media,
as the surviving entity of such merger, will initially become a direct wholly
owned subsidiary of AT&T (the "Four Media Merger");
                               -----------------

          WHEREAS, in connection herewith AT&T, LMC and certain of their
respective affiliates are also entering into a Fifth Amendment to Tax Sharing
Agreement (the "Fifth Tax Sharing Amendment"), dated as of the date hereof,
                ---------------------------
which amends that certain Tax Sharing Agreement, dated as of March 9, 1999, as
amended, (as so amended, the "Tax Sharing Agreement"), to which AT&T, LMC and
                              ---------------------
certain of their respective affiliates are parties;

          WHEREAS, LMC has requested in writing that AT&T enter into the Four
Media Merger Agreement, this Agreement and the Fifth Tax Sharing Agreement and
consummate the transactions contemplated thereby;

          WHEREAS, as a condition to the willingness of each of AT&T and LMC to
enter into the Four Media Merger Agreement, AT&T and the Liberty Media Parties
have determined to enter into this Agreement and the Fifth Tax Sharing
Amendment;

          WHEREAS, AT&T and the Liberty Media Parties desire to supplement and
modify the Inter-Group Agreement in the manner set forth in this Agreement and
to establish in this Agreement certain terms and conditions concerning the
responsibilities and obligations of each

                                       2
<PAGE>

Group to the other as well as certain additional provisions concerning each
Group's relationship with each other as the same may relate to the Four Media
Merger, and certain related transactions;

          WHEREAS, in connection herewith, the Capital Stock Committee has
adopted the resolutions attached as Exhibit A.

          NOW, THEREFORE, in consideration of the foregoing and the mutual
covenants and agreements contained herein, and intending to be legally bound
hereby, AT&T and the Liberty Media Parties hereby agree as follows:

                                   ARTICLE I

                 OBLIGATIONS RELATING TO THE FOUR MEDIA MERGER

          SECTION 1.1.  Post-Closing Contribution.  As soon as reasonably
                        -------------------------
practicable following the effectiveness of the Four Media Merger, each of LMC
and AT&T shall use all reasonable efforts to engage in the Post-Merger
Restructuring Transactions, in each case, to the extent then permitted by law.
Each of AT&T and LMC shall take, and AT&T shall cause each of its Subsidiaries
to take, any and all required actions (whether as a stockholder (or other
interest holder) or through its respective representatives on the board of
directors (or comparable governing body) of the applicable entity), to give
effect to the previous sentence.

          SECTION 1.2.  Certain Liabilities Relating to the Four Media Merger.
                        -----------------------------------------------------

          (a)  Subject to the last sentence of this subsection (a), as between
the Common Stock Group and the Liberty Media Group, all of the following
Liabilities shall be Liabilities of the Liberty Media Group:

          (i)   all Liabilities of Four Media and any of its subsidiaries and
                affiliates (and, in each case, any predecessor or successor
                thereto);

                                       3
<PAGE>

          (ii)  all Liabilities arising out of or related to the Four Media
                Merger Agreement and the transactions contemplated thereby,
                including (A) all Liabilities arising out of or relating to any
                breach of the Four Media Merger Agreement by LMC or Four Media
                (whether such breach is of a representation, warranty, agreement
                or obligation made to AT&T, LMC or otherwise, and without
                reference to any qualification of any such representation,
                warranty, agreement or obligation by reference to materiality,
                any Company SEC Reports or the Company Disclosure Schedule, and
                without reference to any limitation on survival of such
                representation, warranty, covenant or obligation set forth in
                the Four Media Merger Agreement, (B) any Liabilities to third
                parties contemplated by Section 4.25 of the Four Media Merger
                Agreement, (C) all Liabilities required to carry out the
                provisions of Sections 2.1, 2.3 and 2.4 (in each case subject to
                Section 2.5) of the Four Media Merger Agreement, (D) all
                Liabilities under Section 3.7 of the Four Media Merger
                Agreement, (E) all Liabilities arising out of or related to any
                registration rights granted by Four Media (it being agreed that
                AT&T is not responsible for and is not assuming any such
                obligation and shall not be required to register any AT&T
                Liberty Tracking Shares or shares of Parent Common Stock
                pursuant to any registration rights agreement related to Four
                Media or the holders of any securities of Four Media, other than
                the registration of the Stock Consideration pursuant to the
                Registration Statement, as provided in the Four Media Merger
                Agreement), (F) all Liabilities arising out of or related to any

                                       4
<PAGE>

                lawsuit brought, proposed or threatened against AT&T or Merger
                Sub D in connection with the execution, delivery or performance
                of the Four Media Merger Agreement or this Agreement by AT&T or
                Merger Sub D, by any third party other than Four Media, it being
                understood that all Liabilities arising out of or related to any
                such lawsuit brought, proposed or threatened by Four Media shall
                be the Liabilities of the Liberty Media Group if such lawsuit
                relates to any action or omission that (1) is not (or under this
                Agreement would be deemed not to be) a violation of the Four
                Media Merger Agreement or this Agreement or (2) otherwise was
                taken, or omitted to be taken, at the request or direction of
                any member of the Liberty Media Group, including any issuance of
                or failure to issue Class A Liberty Group Stock (regardless of
                whether any such issuance would be in excess of the number of
                shares of Class A Liberty Group Stock that are authorized but
                not issued or reserved for issuance), and (G) any Liabilities
                required to be incurred to relieve AT&T and any member of the
                Common Stock Group of any Liability or obligation that binds or
                is enforceable against AT&T or such member of the Common Stock
                Group as a result of any agreement between Four Media and/or any
                of its Subsidiaries, on the one hand, and any other person, on
                the other hand (other than any such agreement executed by AT&T
                or a member of the Common Stock Group);

          (iii) all Liabilities arising out of the consummation of the Four
                Media Merger or any of the transactions contemplated thereby,
                including the Post-Merger

                                       5
<PAGE>

                Restructuring Transactions, and any other transaction
                contemplated by this Agreement or the Four Media Merger
                Agreement, notwithstanding the failure to receive any consent,
                approval or authorization, or any failure to make any filing or
                notification, from or to any Person that may be required in
                connection therewith;

          (iv)  all Liabilities arising out of the consummation of, or failure
                to consummate, the Post-Merger Restructuring Transactions;

          (v)   all Liabilities relating to current or former employees,
                officers, directors, consultants and other agents of Four Media
                or any of its current or former Subsidiaries and Affiliates,
                including all Company Stock Options assumed by AT&T;

          (vi)  all Liabilities relating to any Warrants assumed by AT&T; and

          (vii) except to the extent resulting from any breach of this Agreement
                or the Four Media Merger Agreement by AT&T that does not arise
                out of or relate to actions taken by AT&T at the request of LMC
                (as contemplated by Section 1.2(d) hereof or otherwise in
                writing), all Liabilities arising out of the execution, delivery
                or performance of this Agreement.

Notwithstanding the foregoing, as between the Common Stock Group and the Liberty
Media Group, the Common Stock Group shall be responsible for (i) any Liabilities
(other than any Liability arising from or relating to a 4MC Tax Item (as defined
in the Fifth Tax Sharing Amendment)) to the extent arising out of or relating to
any breach by AT&T or Merger Sub D of the Four Media Merger Agreement or this
Agreement, that do not arise out of or relate to actions taken or omitted to be
taken

                                       6
<PAGE>

by AT&T or Merger Sub D at the request of LMC (as contemplated by Section 1.2(d)
hereof or otherwise in writing), whether incurred by a member of the Common
Stock Group or by a member of the Liberty Media Group, and (ii) any 4MC Tax Item
to the extent set forth in Sections 3(d)(xiii) and 3(d)(xiv) of the Tax Sharing
Agreement.

          (b)   The Liberty Media Group shall be responsible for, and shall
reimburse the Common Stock Group for, any and all reasonable costs, fees and
expenses incurred by AT&T or any member of the Common Stock Group in connection
with: (i) the negotiation, review, execution and delivery of the Four Media
Merger Agreement, this Agreement and the Fifth Tax Sharing Amendment; (ii) the
consummation of the transactions contemplated by the Four Media Merger Agreement
and this Agreement; (iii) the preparation, review and filing of the Registration
Statement contemplated by the Four Media Merger Agreement (and the Proxy
Statement that will be a part thereof); (iv) the consummation of the Post-Merger
Restructuring Transactions; (v) making any filings with any governmental entity
and otherwise assisting in obtaining approvals, consents and clearances required
in connection with the transactions contemplated by the Four Media Merger
Agreement and this Agreement, if any; and (vi) all other fees and expenses
related thereto, including in each case any internal costs, fees and expenses
(which shall be determined in any reasonable manner developed by AT&T for
tracking such internal costs, fees and expenses), and in each case such costs,
fees and expenses to be reimbursed promptly upon receipt of a statement
therefor.

          (c)   LMC shall indemnify and hold harmless AT&T and each member of
the Common Stock Group from and against, and pay and reimburse AT&T and each
member of the Common Stock Group for, any and all Liabilities (including
reasonable attorneys' fees and expenses)

                                       7
<PAGE>

for which the Liberty Media Group is responsible pursuant to this Section 1.2 in
each case in accordance with Section 1.4(c) of the Inter-Group Agreement.

          (d)   Notwithstanding any provision of the Four Media Merger
Agreement, AT&T and Merger Sub D shall not waive any condition to their
respective obligations to consummate the Four Media Merger set forth in Article
VIII of the Four Media Merger Agreement (the "Parent Closing Conditions")
                                              -------------------------
without LMC's prior written consent; provided, however, that consummation of the
                                     --------  -------
Four Media Merger by Parent based on the reasonable belief that all of the
Parent Closing Conditions applicable to such merger have been satisfied shall
not be deemed a breach of this Agreement unless (and to the extent) AT&T
receives a written request referred to in the next sentence at least 24 hours
prior to the then scheduled Closing of such merger. To the extent so requested
in writing by LMC, each of AT&T and Merger Sub D shall (i) assert that one or
more of the Parent Closing Conditions applicable to the Four Media Merger have
not been satisfied and decline to consummate the transactions contemplated by
the Four Media Merger Agreement or (ii) to the extent lawful, waive one or more
of the Parent Closing Conditions (other than those set forth in Section 8.1 or
Section 8.3(d) of the Four Media Merger Agreement). The Liberty Media Group
agrees to indemnify AT&T for any Liabilities incurred by AT&T or any member of
the Common Stock Group as a result of the taking of an action by AT&T or Merger
Sub D at the request of LMC as contemplated by the previous sentence (but
without regard to whether such action constitutes a breach of the Four Media
Merger Agreement). Notwithstanding clause (ii) of the second sentence of this
paragraph, AT&T shall not be obligated to waive any Parent Closing Condition and
consummate the Four Media Merger if AT&T, in its reasonable judgment, determines
that the foregoing agreement of LMC to indemnify AT&T from and against any
Liability incurred as a result

                                       8
<PAGE>

of such action is not adequate to fully protect AT&T and the Common Stock Group
from and against any Liability that could reasonably be expected to result from
such waiver and the consummation of the Four Media Merger in connection
therewith (it being understood that no such indemnification would be adequate if
such Liability relates to, without limitation, any injunction applicable to AT&T
or any of its Subsidiaries, any restriction on the operation of any business or
assets of AT&T or any of its Subsidiaries or the relationship of AT&T or any of
its Subsidiaries with any governmental entity that has jurisdiction over AT&T or
such Subsidiary or any of their assets or businesses). Except as set forth in
this paragraph, nothing herein shall prohibit AT&T or Merger Sub D from
exercising any of its rights under the Four Media Merger Agreement. AT&T shall
reasonably consider any amendments to, or any waiver of any other provision of,
the Four Media Merger Agreement proposed by LMC. If so requested by AT&T, in
connection with the closing of any Livewire Merger, LMC shall acknowledge in
writing to AT&T that the consummation of such merger would not be inconsistent
with AT&T's obligations under this paragraph, and the failure of AT&T to
consummate such merger in the absence of such acknowledgment shall be deemed,
for all purposes of this Agreement, not to be a violation of this Agreement or
the Four Media Merger Agreement.

          (e)   All employees, officers, directors, consultants and other agents
of Four Media and its Subsidiaries shall be deemed to be employees, officers,
directors, consultants and other agents of the Liberty Media Group, and all
Rollover Options and Rollover Warrants (including any obligations pursuant to
Section 2.3(b) of the Four Media Merger Agreement) that are created as a result
of the Four Media Merger shall be the responsibility of the Liberty Media Group.

                                       9
<PAGE>

          (f)   For purposes of Section 1.16 of the Inter-Group Agreement, the
Rollover Options and the Rollover Warrants to be assumed at the Effective Time
of the Four Media Merger, the obligation to issue Class A Liberty Group Stock to
shareholders and warrant holders of Four Media pursuant to the Four Media Merger
Agreement, and any other obligation to issue or sell any Class A Liberty Group
Stock pursuant to or in connection with the Four Media Merger, the Four Media
Merger Agreement, and any agreement referred to in the Four Media Merger
Agreement or executed in connection therewith (the "Four Media Issuance
                                                    -------------------
Obligations") shall be deemed to be Tracking Stock Obligations (as defined in
-----------
the Inter-Group Agreement) incurred with a Liberty Approval (as defined in the
Inter-Group Agreement), and the incurrence of such obligations shall not
constitute a breach of the second sentence of Section 1.16 of the Inter-Group
Agreement. The issuance of Class A Liberty Group Stock pursuant to the Four
Media Merger and any other transaction contemplated by the Four Media Merger
Agreement shall be deemed to have occurred with a Liberty Approval for purposes
of Section 1.11(a) of the Inter-Group Agreement and shall not be a breach of
Section 1.11(b) or (c) of the Inter-Group Agreement. Subject to the requirements
set forth in Section 1.16 of the Inter-Group Agreement that apply following a
Tax Law Change, any action taken by AT&T to implement the terms of this
paragraph (f) or otherwise to satisfy the Four Media Issuance Obligations shall
not constitute a breach of any provision of the Inter-Group Agreement.

          (g)   At all times from the date of its formation until the Effective
Time of the Four Media Merger, Merger Sub D shall be a direct, wholly owned
Subsidiary of AT&T.

          SECTION 1.3.  Allocation of Proceeds.  For purposes of Sections 1.6(b)
                        ----------------------
and 1.11 of the Inter-Group Agreement and the definition of the term "Liberty
Media Group" in the

                                      10
<PAGE>

AT&T Charter Amendment, the net proceeds of the issuance of the Class A Liberty
Group Stock to be issued in the Four Media Merger shall consist of the entire
interest of AT&T in Four Media and each of its respective Subsidiaries other
than any interest in AT&T Liberty Tracking Shares (the "Excluded Shares")
received in respect of any shares of common stock of Four Media owned by AT&T or
any of its Subsidiaries immediately prior to the Effective Time. Following the
Effective Time, Four Media and its Subsidiaries shall be members of the Liberty
Media Group. For purposes of Section 1.6(b) of the Inter-Group Agreement, all
proceeds received by Four Media upon exercise or conversion of any options,
warrants, debentures or other securities that may be exercised, converted or
exchanged in whole or in part for Class A Liberty Group Stock shall be deemed to
have been received by or contributed to LMC, without any further action by
Parent or any other Person. Notwithstanding anything to the contrary in the
Inter-Group Agreement, there shall be no restriction on the Transfer of any
Excluded Shares and there shall be no obligation to contribute to LMC the net
proceeds from any such Transfer(s).

          In furtherance of the foregoing, any contract right or other similar
right associated with any asset that is attributed to a Group (as defined in the
Inter-Group Agreement) in accordance with the preceding paragraph shall be the
right of the Group (as defined in the Inter-Group Agreement) to which such
related asset is so attributed.

          SECTION 1.4.   Certain Obligations.
                         -------------------
          (a)  AT&T agrees to reasonably consider any actions that it may be
requested by LMC to take in accordance with consummation of the Four Media
Merger.

          (b)  LMC represents and warrants that true and complete copies of all
documents referred to in the Four Media Merger Agreement that have been
delivered to LMC by or on behalf

                                      11
<PAGE>

of Four Media on or prior to the date hereof have been delivered to AT&T on or
prior to the date hereof and agrees that true and complete copies of all such
documents delivered to LMC from and after the date hereof until consummation of
the transactions contemplated hereby will be delivered promptly to AT&T.

          (c)  To the fullest extent possible, LMC will perform all obligations
of AT&T under Sections 3.2 and 3.4(a) of the Four Media Merger Agreement, such
as (to the extent, if any, required of Parent thereby) preparation of documents
and filings, and seeking necessary consents. In connection with the foregoing,
LMC will not make any filing or take any action in the name of or on behalf of
AT&T without receiving the prior written consent of AT&T.

          (d)  AT&T acknowledges that its covenants in this Agreement and the
Four Media Merger Agreement are subject to the fiduciary duties of its board of
directors to all of its stockholders, including the holders of Parent Common
Stock and the holders of AT&T Liberty Tracking Shares, in each case in
accordance with the Parent Charter and the AT&T board of directors' Policy
Statement regarding Liberty Media Group Tracking Stock Matters.

                                  ARTICLE II

                                  DEFINITIONS

          SECTION 2.1.   Certain Definitional Provisions.  Capitalized terms
                         -------------------------------
used herein without definition have the meanings ascribed to such terms in (i)
the Four Media Merger Agreement or (ii) if not otherwise defined herein or in
the Four Media Merger Agreement, the Inter-Group Agreement. The language used in
this Agreement shall be deemed to be the language chosen by the parties to
express their mutual intent, and no rule of strict construction shall be applied
against any party. Any references to any statute or law shall also refer to all
rules and regulations promulgated

                                      12
<PAGE>

thereunder, unless the context requires otherwise. Whenever the context may
require, any pronoun shall include the corresponding masculine, feminine and
neuter forms. The words "include," "includes," and "including" shall be deemed
to be followed by the phrase "without limitation". The words "hereof," "herein"
and "hereunder" and words of similar import when used in this Agreement shall
refer to this Agreement as a whole and not to any particular provision of this
Agreement, and Article and Section references are to this Agreement unless
otherwise specified. The meanings given to terms defined herein shall be equally
applicable to both the singular and plural forms of such terms. Unless the
context shall otherwise require, any references to any agreement or other
instrument or statute or regulation are to it as amended and supplemented from
time to time (and, in the case of a statute or regulation, to any successor
provisions). Any reference in this Agreement to a "day" or number of "days"
(without the explicit qualification of "business") shall be interpreted as a
reference to a calendar day or number of calendar days. If any action or notice
is to be taken or given on or by a particular calendar day, and such calendar
day is not a business day, then such action or notice shall be deferred until,
or may be taken or given on, the next business day.

                                  ARTICLE III

                                 MISCELLANEOUS

          SECTION 3.1.   Notices.  All notices, requests, demands or other
                         -------
communications required by or otherwise with respect to this Agreement shall be
in writing and shall be deemed to have been given to any party when delivered
personally (by courier service or otherwise), when delivered by telecopy and
confirmed by return telecopy, or upon the receipt after being mailed by first-
class mail, postage prepaid and return receipt requested in each case to the
applicable addresses set forth below:

                                      13
<PAGE>

     If to AT&T or any member of the Common Stock Group:

          AT&T Corp.
          295 North Maple Avenue
          Basking Ridge, New Jersey 07920
          Attention:     Vice President-Law
                         and Corporate Secretary
          Facsimile:     (908) 221-6618

     with a copy to:

          Wachtell, Lipton, Rosen & Katz
          51 West 52nd Street
          New York, New York 10019
          Attention:     Richard D. Katcher, Esq.
                         Steven A. Rosenblum, Esq.
                         David M. Silk, Esq.
          Facsimile:     (212) 403-2000

     If to LMC or any member of the Liberty Media Group:

          Liberty Media Corporation
          9197 South Peoria Street
          Englewood, Colorado 80112
          Attention:     Charles Y. Tanabe, Esq.
          Facsimile:     (720) 875-5382

     with a copy to:

          Baker & Botts, L.L.P.
          599 Lexington Avenue
          New York, New York 10022
          Attention:     Elizabeth M. Markowski, Esq.
                         Marc A. Leaf, Esq.
          Facsimile:     (212) 705-5125

or such address as such party shall have designated by notice so given to each
other party.

          SECTION 3.2.   Amendments; No Waivers.  (a) This Agreement shall be
                         ----------------------
amended, changed, supplemented, waived or otherwise modified only by an
instrument in writing

                                      14
<PAGE>

signed by each of AT&T and LMC (and following a Triggering Event (as defined in
the Inter-Group Agreement), Liberty Media Group LLC).

          (b)  No failure or delay by any party in exercising any right, power
or privilege hereunder shall operate as a waiver thereof nor shall any single or
partial exercise thereof preclude any other or further exercise thereof or the
exercise of any other right, power or privilege. The rights and remedies herein
provided shall be cumulative and not exclusive of any rights or remedies
provided by law.

          SECTION 3.3.   Successors and Assigns.  Neither this Agreement nor
                         ----------------------
any of the rights or obligations under this Agreement shall be assigned, in
whole or in part, by any party without the prior written consent of the other
parties hereto; provided, however, that the assignment of its rights and
                --------  -------
obligations under this Agreement by LMC or any Covered Entity (as defined in the
Inter-Group Agreement) to Liberty Media Group LLC in connection with the
transactions contemplated by the Contribution Agreement shall not require the
consent of AT&T. Subject to the foregoing, the provisions of this Agreement
shall be binding upon and inure to the benefit of the parties hereto and their
respective successors and assigns.

          SECTION 3.4.   Governing Law; Consent to Jurisdiction.  This
                         --------------------------------------
Agreement and all disputes hereunder shall be governed by and construed and
enforced in accordance with the internal laws of the State of Delaware, without
regard to the principles of conflicts of laws. Each party hereto irrevocably and
unconditionally consents to submit to the exclusive jurisdiction of the United
States District Court for the District of Delaware or the Chancery Court of the
State of Delaware in any action, suit or proceeding arising in connection with
this Agreement, and agrees that any such action, suit or proceeding shall be
brought only in such court (and waives any objection

                                      15
<PAGE>

based on forum non coveniens or any other objection to venue therein); provided,
         ----- --- ---------                                           --------
however, that such consent to jurisdiction is solely for the purpose referred to
-------
in this Section 3.4 and shall not be deemed to be a general submission to the
jurisdiction of said courts or of the State of Delaware other than for such
purpose. AT&T and LMC each hereby waive any right to a trial by jury in
connection with any such action, suit or proceeding.

          SECTION 3.5.   Counterparts; Effectiveness.  This Agreement may be
                         ---------------------------
executed in any number of counterparts, each of which shall be deemed to be an
original, but all of which together shall constitute one instrument. This
Agreement shall become effective when each party hereto shall have received
counterparts thereof signed by the other party hereto.

          SECTION 3.6.   Specific Performance.  Each of AT&T and LMC
                         --------------------
acknowledges and agrees that money damages are not an effective remedy for
violations of this Agreement and that any party may, in its sole discretion,
apply to a court of competent jurisdiction for specific performance or
injunctive or such other relief as such court may deem just and proper in order
to enforce this Agreement or prevent any violation hereof and, to the extent
permitted by applicable Law, each party waives any objection to the imposition
of such relief.

          SECTION 3.7.   Remedies Cumulative.  All rights, powers and remedies
                         -------------------
provided under this Agreement or otherwise available in respect hereof at law or
in equity shall be cumulative and not alternative, and the exercise or beginning
of the exercise of any thereof by any party shall not preclude the simultaneous
or later exercise of any other such right, power or remedy by such party.

          SECTION 3.8.   Termination.  This Agreement shall remain in full
                         -----------
force and effect until the earlier to occur of (i) such time as no Class A
Liberty Group Stock or Class B Liberty

                                      16
<PAGE>

Group Stock are outstanding and (ii) the termination prior to the Effective Time
of the Four Media Merger Agreement, at which time this Agreement shall terminate
and upon termination, no party shall have any liability or further obligation to
the other under this Agreement, except that the provisions of Section 1.2(a)
(other than subsections 1.2 (a) (i), (v) and (vi)), Section 1.2 (b), Section
1.2(c) (to the extent of any Liabilities described therein that arose at or
prior to the termination of this Agreement or under any provision of this
Agreement that survives such termination) and this Section 3.8 shall survive the
termination of this Agreement; provided, however, that such termination shall
not relieve any party hereto of any liability for any breach of this Agreement
or the Four Media Merger Agreement. No termination of this Agreement shall limit
or otherwise affect the rights or obligations of the parties to the Inter-Group
Agreement.

          SECTION 3.9.   Severability.  In case any provision in this Agreement
                         ------------
shall beheld invalid, illegal or unenforceable in a jurisdiction, such provision
shall be modified or deleted, as to the jurisdiction involved, only to the
extent necessary to render the same valid, legal and enforceable, and the
validity, legality and enforceability of the remaining provisions hereof shall
not in any way be affected or impaired thereby nor shall the validity, legality
or enforceability of such provision be affected thereby in any other
jurisdiction.

          SECTION 3.10.  Cooperation.  Each of AT&T and LMC covenants and
                         -----------
agrees with the other to use its reasonable best efforts to cause each member of
the Common Stock Group and each member of the Liberty Media Group, respectively,
to fulfill each of its respective obligations under this Agreement.

                                      17
<PAGE>

          IN WITNESS WHEREOF, the parties hereto have caused this Agreement to
be duly executed as of the day and year first above written.

                                   AT&T CORP.

                                   By:  /s/ Marilyn J. Wasser
                                      ------------------------------------------
                                      Name: Marilyn J. Wasser
                                      Title: Vice President - Law and Secretary

                                   LIBERTY MEDIA CORPORATION

                                   By:  /s/ Charles Y. Tanabe
                                      ------------------------------------------
                                      Name: Charles Y. Tanabe
                                      Title: Senior Vice President

                                   LIBERTY MEDIA GROUP LLC

                                   By:   /s/ Charles Y. Tanabe
                                      ------------------------------------------
                                      Name: Charles Y. Tanabe
                                      Title: Vice President
<PAGE>

                              Each of the following Covered Entities hereby
                              executes this Agreement as a member of the Liberty
                              Media Group to become a party to this Agreement
                              for so long as it remains a Covered Entity under
                              the applicable provisions of the AT&T Charter
                              Amendment:

                              LIBERTY SP, INC.

                              By:   /s/ Charles Y. Tanabe
                                 -----------------------------------------------
                                 Name: Charles Y. Tanabe
                                 Title: Senior Vice President

                              LIBERTY AGI, INC.

                              By:   /s/ Charles Y. Tanabe
                                 -----------------------------------------------
                                 Name: Charles Y. Tanabe
                                 Title: Senior Vice President

                              LMC INTERACTIVE, INC.

                              By:   /s/ Charles Y. Tanabe
                                 -----------------------------------------------
                                 Name: Charles Y. Tanabe
                                 Title: Senior Vice President

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