<PAGE>   1
                                                                     Exhibit 4.1

                                                                  EXECUTION COPY








                             PAINE WEBBER GROUP INC.


                                       and


                      THE CHASE MANHATTAN BANK, as trustee


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


                          Third Supplemental Indenture



                          Dated as of November 3, 2000

                                       to

                                    Indenture
                          Dated as of December 9, 1996


                                 ---------------
<PAGE>   2
                         Junior Subordinated Debentures



                                    THIRD SUPPLEMENTAL INDENTURE dated as of
                           November 3, 2000, among PAINE WEBBER GROUP INC., a
                           corporation duly organized and existing under the
                           laws of Delaware (the "Company"), having its
                           principal office at 1285 Avenue of the Americas, New
                           York, New York 10019, UBS AMERICAS INC., a
                           corporation duly organized and existing under the
                           laws of Delaware ("UBS Americas"), having its
                           principal office at 677 Washington Boulevard,
                           Stamford, Connecticut 06901, and THE CHASE MANHATTAN
                           BANK, a New York banking corporation, as Trustee (the
                           "Trustee").


                                    RECITALS

                  The Company and the Trustee are parties to an Indenture dated
as of December 9, 1996, as supplemented by the First Supplemental Indenture
dated as of December 9, 1996 and the Second Supplemental Indenture dated as of
March 14, 1997 (as so supplemented, the "Indenture"), relating to the issuance
from time to time by the Company of Debentures. Capitalized terms used, and not
otherwise defined, herein shall have the same meanings given them in the
Indenture.

                  The Company, UBS AG, an Aktiengesellschaft organized under the
laws of Switzerland, and UBS Americas (then known as Neptune Merger Subsidiary,
Inc.), a wholly owned subsidiary of UBS AG, entered into an Agreement and Plan
of Merger dated as of July 12, 2000 (the "Merger Agreement"). Pursuant to the
Merger Agreement, the Company will merge with and into UBS Americas, with UBS
Americas as the surviving corporation (the "Merger"), the Merger to be effective
upon the filing, in the office of the Secretary of State of the State of
Delaware, of a certificate of merger (the "Effective Time").

                  The Company has requested the Trustee to join with it in the
execution and delivery of this third supplemental indenture (this "Third
Supplemental Indenture") in order to supplement and amend the Indenture, by
amending certain provisions thereof, pursuant to which UBS Americas will
expressly assume the due and punctual payment of the principal of (and premium,
if any) and interest on all Debentures and the performance of every covenant of
the Indenture on the part of the Company to be performed or observed (the
"Assumption"), as set forth in Section 10.01(a) of the Indenture.
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                  Section 10.01 of the Indenture provides that the Assumption
must be effected through a supplemental indenture, executed and delivered to the
Trustee, in form satisfactory to the Trustee.

                  Section 9.01(a) of the Indenture provides that a supplemental
indenture may be entered into by the Company and the Trustee, without the
consent of any Holders, to evidence the succession of another corporation to the
Company and assumption by any such successor of the covenants of the Company
contained in the Indenture and in the Debentures.

                  At the request of the Trustee, the Company (a) has furnished
the Trustee with (1) an Officers' Certificate complying with the requirements of
Sections 10.01(c) and 13.06 of the Indenture, stating, among other things, that
the Merger and this Third Supplemental Indenture comply with Article X of the
Indenture and that all conditions precedent provided for in the Indenture
relating to the Merger and this Third Supplemental Indenture have been complied
with and (2) an Opinion of Counsel complying with the requirements of Sections
9.05, 10.01(c) and 13.06 of the Indenture and (b) has delivered to the Trustee a
Board Resolution as required by Section 9.01 of the Indenture authorizing the
execution by the Company of this Third Supplemental Indenture and its delivery
by the Company to the Trustee.

                  All things necessary to make this Third Supplemental Indenture
a valid agreement of the Company, UBS Americas and the Trustee, in accordance
with the terms of the Indenture, and a valid amendment of and supplement to the
Indenture have been done.

                  NOW, THEREFORE, THIS THIRD SUPPLEMENTAL INDENTURE WITNESSETH:

                  For and in consideration of the premises and the purchase of
the Debentures by the Holders thereof, it is mutually agreed, for the equal and
proportionate benefit of all Holders of the Debentures or of series thereof, as
follows:

I.       AMENDMENTS TO THE INDENTURE

                  A. Assumption and Substitution. UBS Americas hereby assumes,
as of the Effective Time, the due and punctual payment of the principal of (and
premium, if any) and interest on all the Debentures and the performance of every
covenant of the Indenture on the part of the Company to be performed or
observed. UBS Americas hereby succeeds to, and is substituted for, the Company
in the Indenture, as of the Effective Time.

II.      NOTICES

                  A. For purposes of Section 13.04 of the Indenture, the address
of UBS Americas is 677 Washington Boulevard, Stamford, Connecticut 06901,
Attention: Secretary.
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                  B. UBS Americas hereby agrees to give the Trustee prompt
notice of the Effective Time.

III.     GENERAL PROVISIONS

                  A. The recitals contained herein shall be taken as the
statements of the Company and UBS Americas, and the Trustee assumes no
responsibility for the correctness of same. The Trustee makes no representation
as to the validity of this Third Supplemental Indenture. The Indenture, as
supplemented and amended by this Third Supplemental Indenture, is in all
respects hereby adopted, ratified and confirmed.

                  B. The Trustee hereby accepts this Third Supplemental
Indenture and agrees to perform the same under the terms and conditions set
forth in the Indenture.

                  C. This instrument may be executed in any number of
counterparts, each of which so executed shall be deemed to be an original, but
all such counterparts shall together constitute but one and the same instrument.

                  D. This Third Supplemental Indenture shall be governed by and
construed in accordance with the laws of the State of New York.
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                  IN WITNESS WHEREOF, the parties hereto have caused this Third
Supplemental Indenture to be duly executed, and their corporate seals to be
hereunto affixed and attested, all as of the day and year first above written.

                                     PAINE WEBBER GROUP INC.,

                                          by  /s/ Authorized Officer
                                             -----------------------------------
[SEAL]                                       Title: Senior Vice President

Attest:


 /s/
---------------------------

                                     UBS AMERICAS INC.,

                                          by  /s/  Louis Eber
                                             -----------------------------------
                                             Title:  Executive Director


[SEAL]                                    by  /s/  Sandra Ward Costin
                                             -----------------------------------
                                             Title:  Director


Attest:




                                     THE CHASE MANHATTAN BANK, as Trustee,

                                          by  /s/ Authorized Officer
                                             -----------------------------------
                                             Title: Vice President
---------------------------


[SEAL]

Attest: