<PAGE>   1
                                                                    Exhibit 4.3e






                                UBS AMERICAS INC.
                             (as successor by merger
                          to Paine Webber Group Inc.),



                                UBS AG, GUARANTOR



                                       AND



                        THE CHASE MANHATTAN BANK, TRUSTEE




                          FOURTH SUPPLEMENTAL INDENTURE


                         Dated: as of December 22, 2000



                           Supplemental to Indenture,
                          dated as of December 9, 1996,

                                 as amended by,
                          First Supplemental Indenture,
                          dated as of December 9, 1996,

                         Second Supplemental Indenture,
                         dated as of March 14, 1997, and

                          Third Supplemental Indenture,
                          dated as of November 3, 2000

                                   relating to
                         Junior Subordinated Debentures
<PAGE>   2
                  FOURTH SUPPLEMENTAL INDENTURE, dated as of December 22, 2000,
among UBS AMERICAS INC. (as successor by merger to Paine Webber Group Inc.), a
Delaware corporation (the "Company"), UBS AG, a Swiss banking corporation ("the
Guarantor"), and THE CHASE MANHATTAN BANK, a New York banking corporation (the
"Trustee").

                             RECITALS OF THE COMPANY

                  Pursuant to the Third Supplemental Indenture, dated as of
November 3, 2000 between Paine Webber Group Inc. ("PaineWebber") and the
Company, the Company succeeded to the obligations of PaineWebber under the
Indenture, dated as of December 9, 1996, between PaineWebber and the Trustee, 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 amended by
such Supplemental Indentures, the "Junior Subordinated Indenture"), pursuant to
which PaineWebber issued its outstanding unsecured debentures, notes or other
evidences of indebtedness (the "Outstanding Junior Subordinated Notes").

                  The Company has requested the Trustee and the Guarantor to
join with it in the execution and delivery of this fourth supplemental indenture
(the "Fourth Supplemental Indenture") in order to supplement and amend the
Junior Subordinated Indenture, by amending and adding certain provisions
thereof, to permit the Company to accept the guarantee of the Guarantor of
certain of the Company's obligations.

                  The Company is a wholly owned subsidiary of the Guarantor, and
the Guarantor wishes to guarantee, on a subordinated basis, the Company's
obligations pursuant to the Junior Subordinated Indenture and the Outstanding
Junior Subordinated Notes.

                  The Company has authorized the execution and delivery of this
Fourth Supplemental Indenture by a Board Resolution, as defined in the Junior
Subordinated Indenture.

                  The Guarantor has authorized the execution and delivery of
this Fourth Supplemental Indenture by a
<PAGE>   3
resolution of or under the authority of its board of directors.

                  Section 9.01 of the Junior Subordinated Indenture provides
that a supplemental indenture may be entered into by the Company and the
Trustee, without the consent of any Holders of the Outstanding Junior
Subordinated Notes, to make any provisions with respect to matters or questions
arising under the Junior Subordinated Indenture, provided such action does not
adversely affect the interests of the Holders of Outstanding Junior Subordinated
Notes of any series in any material respect.

                  The Company has determined that the execution and delivery of
this Fourth Supplemental Indenture by the Company, the Guarantor and the Trustee
complies with said Section 9.01 and does not require the consent of any Holder
of the Outstanding Junior Subordinated Notes.

                  At the request of the Trustee, the Company has furnished the
Trustee with an Opinion of Counsel complying with the requirements of Section
9.05 of the Junior Subordinated Indenture, stating, among other things, that the
execution of this Fourth Supplemental Indenture is authorized or permitted by
the Junior Subordinated Indenture, and an Officers' Certificate and Opinion of
Counsel complying with the requirements of Section 13.06 of the Junior
Subordinated Indenture, and has delivered to the Trustee a Board Resolution as
required by Section 9.01 of the Junior Subordinated Indenture authorizing the
execution by the Company of this Fourth Supplemental Indenture and its delivery
by the Company to the Trustee.

                  All conditions and requirements necessary to make this Fourth
Supplemental Indenture a valid agreement of the Company, in accordance with the
terms of the Junior Subordinated Indenture, and a valid amendment of and
supplement to the Junior Subordinated Indenture have been done.

                  NOW THEREFORE, the Company, the Guarantor and the Trustee
hereby agree as follows:

I.  Amendments to the Junior Subordinated Indenture
<PAGE>   4
                  1.       Section 1.01 of the Junior Subordinated Indenture is
hereby amended by the insertion of the following definitions in the appropriate
alphabetical order:

                           "Guarantor" means UBS AG, a Swiss banking
         corporation, until a successor Person shall have become such pursuant
         to the applicable provisions of this Indenture and thereafter
         "Guarantor" shall mean such successor Person.

                           "Guarantor Order" means a written order signed in the
         name of the Guarantor by its Chairman of the Board, its President or a
         Vice President, and by its Treasurer, an Assistant Treasurer, its
         Controller, an Assistant Controller, its Secretary or an Assistant
         Secretary, and delivered to the Trustee.

                           "Guarantee" means the Guarantor's unconditional
         guarantee, on a subordinated basis, of the payment of the Debentures,
         as provided in Article Three-A.

                  2.       The definition of "Officer's Certificate" in Section
1.01 of the Junior Subordinated Indenture is hereby amended and restated in its
entirety as follows:

                           "Officers' Certificate" means, (i) with respect to
         the Company, a certificate signed by the Chairman of the Board, the
         Vice Chairman of the Board, the President or any Vice President, and by
         the Treasurer, the Controller, the Secretary or any Assistant
         Treasurer, Assistant Controller or Assistant Secretary, of the Company,
         and delivered to the Trustee; and (ii) with respect to the Guarantor, a
         certificate signed two persons authorized to bind the Guarantor, and
         delivered to the Trustee. Each such Officers' Certificate shall contain
         the statements provided in Section 13.06, if applicable.

                  3.       The definition of "Opinion of Counsel" in Section
1.01 of the Junior Subordinated Indenture is hereby amended and restated in its
entirety as follows:

                           "Opinion of Counsel" means a written opinion of
         counsel, who may be counsel for or an employee of


                                      -3-
<PAGE>   5
         the Company or the Guarantor and who shall be acceptable to the
         Trustee. Each Opinion of Counsel shall contain the statements provided
         in Section 13.06, if applicable.

                  4.       The Junior Subordinated Indenture is hereby amended
by the insertion of the following Article Three-A after the existing Article
Three:

                                 ARTICLE THREE-A

                                  The Guarantee

Section 3.01-A.  Unconditional Guarantee; Subordination.

                  For value received, the Guarantor hereby unconditionally and
irrevocably guarantees, as a primary obligor and not merely as a surety, for the
benefit of the Holders of the Debentures, and the obligations of the Company,
under the Indenture and the Debentures, including the due and punctual payment
of the principal of (and premium, if any) and interest on the Debentures
(including any additional interest or other amounts payable in accordance with
the terms of the Debentures) together with any other amount as may be owed from
time to time by the Company under the Indenture, when and as the same shall
become due and payable, whether at maturity, upon acceleration, redemption or
otherwise in accordance with the terms of the Debentures. UBS hereby waives
presentment, demand for payment, protest and acceptance of this guarantee and
all notices to any person (whether of nonpayment, dishonor, protest, extension
of credit to the debtor or of other matters referred to in the Indenture or
otherwise) with respect to the Debentures or the Indenture, or any other related
agreement or instrument. If the Company fails punctually to make any timely
payment under the Debentures, the Guarantor hereby agrees to cause any such
payment to be made punctually when and as the same shall become due and payable,
whether at the stated maturity or by declaration of acceleration, or otherwise,
and as if such payment were made by the Company.

                  The Guarantor hereby agrees that this guarantee is an
absolute, present and continuing guarantee of payment and not of collectability
and that its obligations hereunder


                                      -4-
<PAGE>   6
shall be unconditional, irrespective of the validity, legality or enforceability
of the Debentures or the Indenture, the absence of any action to enforce the
same or to collect from the Company, any waiver or consent by the holder of the
Debentures with respect to the provisions thereof, and the recovery of any
judgment against the Company or any action to enforce the same or any other
circumstance that might otherwise result in a legal or equitable discharge or
defense of a guarantor.

                  The Guarantor shall be subrogated to all rights of the Holders
of any Debenture against the Company in respect of any amounts paid by the
Guarantor pursuant to the provisions of the Guarantee; provided, however, that
the Guarantor shall not be entitled to enforce or to receive any payments
arising out of, or based upon, such right of subrogation until the principal of
and interest on all Securities issued hereunder shall have been paid in full.

                  This guarantee shall bind the Guarantor and its successors and
assigns. This guarantee constitutes a direct, unconditional and unsecured
obligation of the Guarantor. The obligations of the Guarantor hereunder will be
subordinated in right of payment to the prior payment in full of the deposit
liabilities of the Guarantor and all other liabilities of the Guarantor
(including all deposit liabilities and other liabilities of the head office and
all offices of the Guarantor wherever located), except (i) any liabilities which
by their terms rank pari passu with or are subordinated to the obligations of
the Guarantor under this guarantee; (ii) any liabilities which by their terms
rank pari passu with or are subordinated to liabilities which by their terms
rank pari passu with or are subordinated to the obligations of the Guarantor
under this guarantee; and (iii) any Existing Junior Subordinated Obligations.
For the avoidance of doubt, the obligations of the Guarantor hereunder will be
senior to any Existing Junior Subordinated Obligations and any liabilities which
by their terms are subordinated to the obligations of the Guarantor under this
guarantee.

                  5. Section 6.01 of the Junior Subordinated Indenture is hereby
amended by inserting the words "or the Guarantor" after the word "Company" in
subparagraph (1) thereof.


                                      -5-
<PAGE>   7
                  6. Section 6.02(b) of the Junior Subordinated Indenture is
hereby amended by inserting a comma and then the words "the Guarantor" following
the word "Company" the second and third times the word "Company" appears.

                  7. Section 6.02(c) of the Junior Subordinated Indenture is
hereby amended by (i) inserting a comma and then the words "the Guarantor" the
first and second times the word "Company" appears, in each case following the
word "Company" and (ii) inserting the words "or the Guarantor" the third time
the word "Company" appears, following the word "Company".

                  8. Section 6.08 of the Junior Subordinated Indenture is hereby
amended by inserting the words "and the Guarantor each" following the word
"Company" in each case where the word "Company" appears.

                  9. Section 7.06 of the Junior Subordinated Indenture is hereby
amended by (i) inserting the words "and the Guarantor" following the word
"Company" in the first line of subparagraph (a), (ii) replacing the word
"covenants" with the word "covenant" in the first line of subparagraph (a),
(iii) replacing the word "agrees" with the word "agree" in the first line of
subparagraph (a), and (iv) inserting the words "and the Guarantor" in
subparagraph (b) following the word "Company".

                  10.      The Junior Subordinated Indenture is hereby amended
by the insertion of the following paragraphs at the end of Article Ten:

Section 10.03. Guarantor May Consolidate, Etc., Only on Certain Terms.

                  The Guarantor shall not consolidate with or merge into any
other Person or convey, transfer or lease its properties and assets
substantially as an entirety to any Person, and the Guarantor shall not permit
any Person to consolidate with or merge into the Guarantor or convey, transfer
or lease its properties and assets substantially as an entirety to the
Guarantor, unless:


                                      -6-
<PAGE>   8
                  (1) in case the Guarantor shall consolidate with or merge into
         another Person or convey, transfer or lease its properties and assets
         substantially as an entirety to any Person, the Person formed by such
         consolidation or into which the Guarantor is merged or the Person which
         acquires by conveyance or transfer, or which leases, the properties and
         assets of the Guarantor substantially as an entirety shall be a
         corporation organized and existing under the laws of Switzerland or any
         other country, or any state or subdivision thereof or therein, and
         shall expressly assume, by an indenture supplemental hereto, executed
         and delivered to the Trustee, in form satisfactory to the Trustee, the
         performance of the Guarantee, and the performance of every covenant of
         this Indenture on the part of the Guarantor to be performed or
         observed;

                  (2) immediately after giving effect to such transaction, and
         treating any indebtedness which becomes an obligation of the Company or
         a Subsidiary as a result of such transaction as having been incurred by
         the Company or such Subsidiary at the time of such transaction, no
         Event of Default, and no event which, after notice or lapse of time or
         both, would become an Event of Default, shall have happened and be
         continuing; and

                  (3) if a supplemental indenture is required in connection with
         such transaction, the Guarantor shall have delivered to the Trustee an
         Officers' Certificate and an Opinion of Counsel, each stating that such
         consolidation, merger, conveyance, transfer or lease and such
         supplemental indenture comply with this Article and that all conditions
         precedent herein provided for relating to such transaction have been
         complied with.


                                      -7-
<PAGE>   9
Section 10.04. Successor Substituted.

                  Upon any consolidation of the Guarantor with, or merger by the
Guarantor into, any other Person or any conveyance, transfer or lease of the
properties and assets of the Guarantor substantially as an entirety in
accordance with Section 10.03, the successor Person formed by such consolidation
or into which the Guarantor is merged or to which such conveyance, transfer or
lease is made shall succeed to, and be substituted for, and may exercise every
right and power of, the Guarantor under this Indenture with the same effect as
if such successor Person had been named as the Guarantor herein, and thereafter,
except in the case of a lease, the predecessor Person shall be relieved of all
obligations and covenants under this Indenture and the Guarantee.

                  11. Section 9.01 of the Junior Subordinated Indenture is
hereby amended by inserting the words "or the Guarantor" after the word
"Company" subparagraph (a) thereof in each case where the word "Company"
appears.

                  12. Section 4.03(c) of the Junior Subordinated Indenture is
hereby amended by (i) inserting the words "or the Guarantor" after the words
"the Company" at the beginning of clause (ii) of such subparagraph, and (ii)
inserting the words "or Guarantor Order, as the case may be" after the words
"Company Order."

                  13. Ratification and Confirmation. As amended and modified by
this Fourth Supplemental Indenture, the Junior Subordinated Indenture is in all
respects ratified and confirmed and the Junior Subordinated Indenture and this
Fourth Supplemental Indenture shall be read, taken and construed as one and the
same instrument.

                  14. Counterparts. This Fourth Supplemental Indenture may be
executed in any number of counterparts and all said counterparts executed and
delivered each as an original shall constitute but one and the same instrument.

                  15. Trustee's Duties, Responsibilities and Liabilities. The
recitals contained herein shall be taken as the statements of the Company and
the Guarantor, and the Trustee assumes no responsibility for the correctness of


                                      -8-
<PAGE>   10
same. The Trustee makes no representation as to the validity of this Fourth
Supplemental Indenture. The Trustee assumes no duties, responsibilities or
liabilities by reason of this Fourth Supplemental Indenture other than as set
forth in the Junior Subordinated Indenture, and this Fourth Supplemental
Indenture is executed and accepted by the Trustee subject to all the terms and
conditions of their acceptance of the trust under the Junior Subordinated
Indenture, as fully as if said terms and conditions were herein set forth at
length.


                                       -9-
<PAGE>   11
                  IN WITNESS WHEREOF, the parties hereto have caused this Fourth
Supplemental Indenture to be duly executed, and their respective corporate seals
to be hereunto affixed and attested, as of the day and year first above written.

                                  UBS AMERICAS INC.
                                  (as successor by merger to
                                  Paine Webber Group Inc.)



                                  By: ______________________________
                                      Name:
                                      Title:

ATTEST:


_______________________________
Name:
Title:

                                  UBS AG



                                  By: ______________________________
                                      Name:
                                      Title:

ATTEST:


_______________________________
Name:
Title:

                                  THE CHASE MANHATTAN BANK



                                  By: ______________________________
                                      Name:
                                      Title:
ATTEST:


_______________________________
Name:
Title: