<PAGE>
                                                                 EXHIBIT 4(b)(4)

                             PUBLIC SERVICE COMPANY
                                   OF COLORADO

                                       TO


                      U.S. BANK TRUST NATIONAL ASSOCIATION,

                                                                      AS TRUSTEE


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


                         SUPPLEMENTAL INDENTURE NO. [ ]

                          Dated as of [              ]
                                       --------------

                          Supplemental to the Indenture
                           dated as of October 1, 1993

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


                 Establishing the Securities of Series No. [ ],
             designated First Collateral Trust Bonds, Series No. [ ]



<PAGE>

         SUPPLEMENTAL INDENTURE NO. [ ], dated as of [______________], between
PUBLIC SERVICE COMPANY OF COLORADO, a corporation duly organized and existing
under the laws of the State of Colorado (hereinafter sometimes called the
"Company"), and U.S. BANK TRUST NATIONAL ASSOCIATION (FORMERLY FIRST TRUST OF
NEW YORK, NATIONAL ASSOCIATION), a national banking association, as successor
trustee (hereinafter sometimes called the "Trustee") to Morgan Guaranty Trust
Company of New York under the Indenture, dated as of October 1, 1993
(hereinafter called the "Original Indenture"), as previously supplemented and as
further supplemented by this Supplemental Indenture No. [ ]. The Original
Indenture and any and all indentures and all other instruments supplemental
thereto are hereinafter sometimes collectively called the "Indenture".

                             RECITALS OF THE COMPANY

         The Original Indenture was authorized, executed and delivered by the
Company to provide for the issuance from time to time of its Securities (such
term and all other capitalized terms used herein without definition having the
meanings assigned to them in the Original Indenture), to be issued in one or
more series as contemplated therein, and to provide security for the payment of
the principal of and premium, if any, and interest, if any, on the Securities.

         The Company has heretofore executed and delivered to the Trustee the
Supplemental Indentures referred to in Schedule A hereto for the purpose of
establishing a series of bonds and appointing the successor Trustee.

         The Company desires to establish a series of Securities to be
designated "First Collateral Trust Bonds, Series No. [ ]", such series of
Securities to be hereinafter sometimes called "Series No. [ ]".

         The Company has duly authorized the execution and delivery of this
Supplemental Indenture No. [ ] to establish the Securities of Series No. [ ] and
has duly authorized the issuance of such Securities; and all acts necessary to
make this Supplemental Indenture No. [ ] a valid agreement of the Company, and
to make the Securities of Series No. [ ] valid obligations of the Company, have
been performed.

                                GRANTING CLAUSES

         NOW, THEREFORE, THIS SUPPLEMENTAL INDENTURE NO. [ ] WITNESSETH, that,
in consideration of the premises and of the purchase of the Securities by the
Holders thereof, and in order to secure the payment of the principal of and
premium, if any, and interest, if any, on all Securities from time to time
Outstanding and the performance of the covenants contained therein and in the
Indenture and to declare the terms and conditions on which such Securities are
secured, the Company hereby grants, bargains, sells, releases, conveys, assigns,
transfers, mortgages, pledges, sets over and confirms to the Trustee, and grants
to the Trustee a security interest in, the following:

                              GRANTING CLAUSE FIRST

         All right, title and interest of the Company, as of the date of the
         execution and delivery of this Supplemental Indenture No. [ ], in and
         to property (other than Excepted Property), real, personal and mixed
         and wherever situated, in any case used or to be used in or in
         connection with the Electric Utility Business (whether or not such use
         is the sole use of such property), including without limitation (a) all
         lands, easements, servitudes, licenses, permits, rights of way and
         other rights and interests in or relating to real property used or to
         be used in or in connection with the Electric Utility Business or
         relating to the occupancy or use of such real property, subject
         however, to the exceptions and exclusions set forth in clause (a) of
         Granting Clause First of the Original Indenture; (b) all plants,


<PAGE>


         generators, turbines, engines, boilers, fuel handling and
         transportation facilities, air and water pollution control and sewage
         and solid waste disposal facilities and other machinery and facilities
         for the generation of electric energy; (c) all switchyards, lines,
         towers, substations, transformers and other machinery and facilities
         for the transmission of electric energy; (d) all lines, poles,
         conduits, conductors, meters, regulators and other machinery and
         facilities for the distribution of electric energy; (e) all buildings,
         offices, warehouses and other structures used or to be used in or in
         connection with the Electric Utility Business; (f) all pipes, cables,
         insulators, ducts, tools, computers and other data processing and/or
         storage equipment and other equipment, apparatus and facilities used or
         to be used in or in connection with the Electric Utility Business; (g)
         any or all of the foregoing properties in the process of construction;
         and (h) all other property, of whatever kind and nature, ancillary to
         or otherwise used or to be used in conjunction with any or all of the
         foregoing or otherwise, directly or indirectly, in furtherance of the
         Electric Utility Business;

                             GRANTING CLAUSE SECOND

         Subject to the applicable exceptions permitted by Section 810(c),
         Section 1303 and Section 1305 of the Original Indenture, all property
         (other than Excepted Property) of the kind and nature described in
         Granting Clause First which may be hereafter acquired by the Company,
         it being the intention of the Company that all such property acquired
         by the Company after the date of the execution and delivery of this
         Supplemental Indenture No. [ ] shall be as fully embraced within and
         subjected to the Lien hereof as if such property were owned by the
         Company as of the date of the execution and delivery of this
         Supplemental Indenture No. [ ];

                             GRANTING CLAUSE FOURTH

         All other property of whatever kind and nature subjected or required to
         be subjected to the Lien of the Indenture by any of the provisions
         thereof;

                                EXCEPTED PROPERTY

         Expressly excepting and excluding, however, from the Lien and operation
         of the Indenture all Excepted Property of the Company, whether now
         owned or hereafter acquired;

         TO HAVE AND TO HOLD all such property, real, personal and mixed, unto
the Trustee, its successors in trust and their assigns forever;

         SUBJECT, HOWEVER, to (a) Liens existing at the date of the execution
and delivery of the Original Indenture (including, but not limited to, the Lien
of the PSCO 1939 Mortgage), (b) as to property acquired by the Company after the
date of the execution and delivery of the Original Indenture, Liens existing or
placed thereon at the time of the acquisition thereof (including, but not
limited to, the Lien of any Class A Mortgage and purchase money Liens), (c)
Retained Interests and (d) any other Permitted Liens, it being understood that,
with respect to any property which was at the date of execution and delivery of
the Original Indenture or thereafter became or hereafter becomes subject to the
Lien of any Class A Mortgage, the Lien of the Indenture shall at all times be
junior, subject and subordinate to the Lien of such Class A Mortgage;


                                       2
<PAGE>


         IN TRUST, NEVERTHELESS, for the equal and proportionate benefit and
security of the Holders from time to time of all Outstanding Securities without
any priority of any such Security over any other such Security;

         PROVIDED, HOWEVER, that the right, title and interest of the Trustee in
and to the Mortgaged Property shall cease, terminate and become void in
accordance with, and subject to the conditions set forth in, Article Nine of the
Original Indenture, and if, thereafter, the principal of and premium, if any,
and interest, if any, on the Securities shall have been paid to the Holders
thereof, or shall have been paid to the Company pursuant to Section 603 of the
Original Indenture, then and in that case the Indenture shall terminate, and the
Trustee shall execute and deliver to the Company such instruments as the Company
shall require to evidence such termination; otherwise the Indenture, and the
estate and rights thereby granted shall be and remain in full force and effect;
and

         THE PARTIES HEREBY FURTHER COVENANT AND AGREE as follows:

                                   ARTICLE ONE

                          SECURITIES OF SERIES NO. [ ]

         There are hereby established the Securities of Series No. [ ], which
shall have the terms and characteristics set forth below (the lettered
subdivisions set forth below corresponding to the lettered subdivisions of
Section 301 of the Original Indenture):

         (a) the title of the Securities of such series shall be "First
         Collateral Trust Bonds, Series No. [ ]"; provided, however, that, at
         any time after the PSCO 1939 Mortgage shall have been satisfied and
         discharged, the Company shall have the right, without any consent or
         other action by the Holders of such Securities, to change such title in
         such manner as shall be deemed by the Company to be appropriate to
         reflect such satisfaction and discharge, such change to be evidenced in
         an Officer's Certificate;

         (b) the Securities of Series No. [ ] shall be initially authenticated
         and delivered in the aggregate principal amount of [up to]
         $[___________];

         (c) interest on the Securities of Series No. [ ] shall be payable to
         the Persons in whose names such Securities are registered at the close
         of business on the Regular Record Date for such interest, except as
         otherwise expressly provided in the form of such Securities attached as
         Exhibit A hereto;

         (d) the principal of the Securities of Series No. [ ] shall be payable
         on [ ], the Stated Maturity.

         (e) the Securities of Series No. [ ] shall bear interest at a rate of
         [ ]% per annum; interest shall accrue on the Securities of Series No.
         [ ] from [______________], or the most recent date to which interest
         has been paid or duly provided for; the Interest Payment Dates for such
         Securities shall be [ ] and [ ] in each year, commencing
         [______________], and the Regular Record Dates with respect to the
         Interest Payment Dates for such Securities shall be [ ] and [ ] in each
         year, respectively (whether or not a Business Day);


                                       3
<PAGE>


         (f) the Corporate Trust Office of U.S. Bank Trust National Association
         in New York, New York shall be the place at which (i) the principal
         of[, premium, if any,] and interest, if any, on the Securities of
         Series No. [ ] shall be payable, (ii) registration of transfer of such
         Securities may be effected, (iii) exchanges of such Securities may be
         effected and (iv) notices and demands to or upon the Company in respect
         of such Securities and the Indenture may be served; and U.S. Bank Trust
         National Association shall be the Security Registrar for such
         Securities; provided, however, that the Company reserves the right to
         change, by one or more Officer's Certificates, any such place or the
         Security Registrar; and provided, further, that the Company reserves
         the right to designate, by one or more Officer's Certificates, its
         principal office in Denver, Colorado as any such place or itself as the
         Security Registrar;

         (g) the Securities of Series No. [ ] [shall not be redeemable prior to
         [____________]. On and after that date, the Securities of Series No.
         [ ]]; [shall be redeemable in whole or in part, at the option of the
         Company, during the period, at [__________________].

         (h) not applicable;

         (i) not applicable;

         (j) not applicable;

         (k) not applicable;

         (l) not applicable;

         (m) not applicable;

         (n) not applicable;

         (o) not applicable;

         (p) not applicable;

         (q) the Securities of Series No. [ ] are to be initially registered in
         the name of Cede & Co., as nominee for The Depository Trust Company
         (the "Depositary"). Such Securities shall not be transferable or
         exchangeable, nor shall any purported transfer be registered, except as
         follows:

                  (i)      such Securities may be transferred in whole, and
                           appropriate registration of transfer effected, if
                           such transfer is by such nominee to the Depositary,
                           or by the Depositary to another nominee thereof, or
                           by any nominee of the Depositary to any other nominee
                           thereof, or by the Depositary or any nominee thereof
                           to any successor securities depositary or any nominee
                           thereof; and

                  (ii)     such Securities may be exchanged for definitive
                           Securities registered in the respective names of the
                           beneficial holders thereof, and thereafter shall be
                           transferable without restriction, if:

                           (A)      the Depositary, or any successor securities
                                    depositary, shall have notified the Company
                                    and the Trustee that it is unwilling or
                                    unable to continue to act as securities
                                    depositary with respect to such Securities
                                    or the Depository has ceased to be a
                                    clearing agency registered under the


                                       4
<PAGE>


                                    Securities Exchange Act of 1934, as amended,
                                    and the Trustee shall not have been notified
                                    by the Company within ninety (90) days of
                                    the identity of a successor securities
                                    depositary with respect to such Securities;

                           (B)      the Company shall have delivered to the
                                    Trustee a Company Order to the effect that
                                    such Securities shall be so exchangeable on
                                    and after a date specified therein; or

                           (C)      (1) an Event of Default shall have occurred
                                    and be continuing, (2) the Trustee shall
                                    have given notice of such Event of Default
                                    pursuant to Section 1102 of the Original
                                    Indenture and (3) there shall have been
                                    delivered to the Company and the Trustee an
                                    Opinion of Counsel to the effect that the
                                    interests of the beneficial owners of such
                                    Securities in respect thereof will be
                                    materially impaired unless such owners
                                    become Holders of definitive Securities.;

         (r) not applicable;

         (s) no service charge shall be made for the registration of transfer or
         exchange of the Securities of Series No. [ ]; provided, however, that
         the Company may require payment of a sum sufficient to cover any tax or
         other governmental charge payable in connection with the exchange or
         transfer;

         (t) not applicable;

         (u) (i) If the Company shall have caused the Company's indebtedness in
             respect of any Securities of Series No. [ ] to have been satisfied
             and discharged prior to the Maturity of such Securities, as
             provided in Section 901 of the Original Indenture, the Company
             shall, promptly after the date of such satisfaction and discharge,
             give a notice to each Person who was a Holder of any of such
             Securities on such date stating (A)(1) the aggregate principal
             amount of such Securities and (2) the aggregate amount of any money
             (other than amounts, if any, deposited in respect of accrued
             interest on such Securities) and the aggregate principal amount of,
             the rate or rates of interest on, and the aggregate fair market
             value of, any Eligible Obligations deposited pursuant to Section
             901 of the Original Indenture with respect to such Securities and
             (B) that the Company will provide (and the Company shall promptly
             so provide) to such Person, or any beneficial owner of such
             Securities holding through such Person (upon written request to the
             Company sent to an address specified in such notice), such other
             information as such Person or beneficial owner, as the case may be,
             reasonably may request in order to enable it to determine the
             federal income tax consequences to it resulting from the
             satisfaction and discharge of the Company's indebtedness in respect
             of such Securities. Thereafter, the Company shall, within
             forty-five (45) days after the end of each calendar year, give to
             each Person who at any time during such calendar year was a Holder
             of such Securities a notice containing (X) such information as may
             be necessary to enable such Person to report its income, gain or
             loss for federal income tax purposes with respect to such
             Securities or the assets held on deposit in respect thereof during
             such calendar year or the portion thereof during which such Person
             was a Holder of such Securities, as the case may be (such
             information to be set forth for such calendar year as a whole and
             for each month during such year) and (Y) a statement to the effect
             that the Company will provide (and the Company shall promptly so
             provide) to such Person, or any beneficial owner of


                                       5
<PAGE>


             such Securities holding through such Person (upon written request
             to the Company sent to an address specified in such notice), such
             other information as such Person or beneficial owner, as the case
             may be, reasonably may request in order to enable it to determine
             its income, gain or loss for federal income tax purposes with
             respect to such Securities or such assets for such year or portion
             thereof, as the case may be. The obligation of the Company to
             provide or cause to be provided information for purposes of income
             tax reporting by any Person as described in the first two sentences
             of this paragraph shall be deemed to have been satisfied to the
             extent that the Company has provided or caused to be provided
             substantially comparable information pursuant to any requirements
             of the Internal Revenue Code of 1986, as amended from time to time
             (the "Code") and United States Treasury regulations thereunder.

             (ii) Notwithstanding the provisions of subparagraph (i) above, the
             Company shall not be required to give any notice specified in such
             subparagraph or to otherwise furnish any of the information
             contemplated therein if the Company shall have delivered to the
             Trustee an Opinion of Counsel to the effect that the Holders of
             such Securities will not recognize income, gain or loss for federal
             income tax purposes as a result of the satisfaction and discharge
             of the Company's indebtedness in respect of such Securities and
             such Holders will be subject to federal income taxation on the same
             amounts and in the same manner and at the same times as if such
             satisfaction and discharge had not occurred.

             (iii) Anything in this clause (u) to the contrary notwithstanding,
             the Company shall not be required to give any notice specified in
             subparagraph (i) or to otherwise furnish the information
             contemplated therein or to deliver any Opinion of Counsel
             contemplated by subparagraph (ii) if the Company shall have caused
             Securities of Series No. [ ] to be deemed to have been paid for
             purposes of the Indenture, as provided in Section 901 of the
             Original Indenture, but shall not have effected the satisfaction
             and discharge of its indebtedness in respect of such Securities
             pursuant to such Section.

         (v) The Securities of Series No. [ ] shall be substantially in the form
         attached hereto as Exhibit A and shall have such further terms as are
         set forth in such form.

                                   ARTICLE TWO

                            MISCELLANEOUS PROVISIONS

         This Supplemental Indenture No. [ ] is a supplement to the Original
Indenture. As previously supplemented and further supplemented by this
Supplemental Indenture No. [ ], the Original Indenture is in all respects
ratified, approved and confirmed, and the Original Indenture, all previous
supplements thereto and this Supplemental Indenture No. [ ] shall together
constitute one and the same instrument.


                                       6
<PAGE>


         IN WITNESS WHEREOF, the parties hereto have caused this Supplemental
Indenture No. [ ] to be duly executed as of the day and year first above
written.


                                     PUBLIC SERVICE COMPANY OF COLORADO



                                     By:
                                            ------------------------------------

                                            Name:
                                            Title:




STATE OF [__________]       )
                            ) ss.:
COUNTY OF [__________]      )

         On the [_____] day of [________, ______], before me personally came
[__________] to me known, who, being by me duly sworn, did depose and say that
he is a [__________] of Public Service Company of Colorado, one of the
corporations described in and which executed the foregoing instrument; and that
he signed his name thereto by authority of the Board of Directors of said
corporation.

                                      ------------------------------------------
                                      Name:
                                      Notary Public, State of [          ]
                                                               ----------
                                      Commission Expires:


                                       7
<PAGE>


                                     U.S. BANK TRUST NATIONAL ASSOCIATION,
                                                            Trustee



                                     By:
                                        ----------------------------------------

                                        Name:
                                        Title:



STATE OF [__________]            )
                                 ) ss.:
COUNTY OF [__________]           )

         On the [_____] day of [__________, ____], before me personally came
[__________], to me known, who, being by me duly sworn, did depose and say that
he is an [__________] of U.S. Bank Trust National Association, the banking
association described in and which executed the foregoing instrument; and that
he signed his name thereto by authority of the Board of Directors of said
banking association.

                                      ------------------------------------------
                                      Name:
                                      Notary Public, State of [          ]
                                                               ----------
                                      Commission Expires:


                                       8
<PAGE>


                                                                       EXHIBIT A

                                FORM OF SECURITY


                   (See legend at the end of this Security for
                            restrictions on transfer)


                       PUBLIC SERVICE COMPANY OF COLORADO
                   First Collateral Trust Bond, Series No. [ ]



       Original Interest Accrual Date:           [                          ]
                                                  --------------------------
                        Interest Rate:           [                          ]
                                                  --------------------------
                      Stated Maturity:           [                          ]
                                                  --------------------------
               Interest Payment Dates:           [            ] and [       ]
                                                  -------------       ------
                 Regular Record Dates:           [            ] and [       ]
                                                  -------------       ------


                    This Security is not a Discount Security
              within the meaning of the within-mentioned Indenture

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


Principal Amount                                                  Registered No.
$                                                                 CUSIP


         PUBLIC SERVICE COMPANY OF COLORADO, a corporation duly organized and
existing under the laws of the State of Colorado (herein called the "Company,"
which term includes any successor corporation under the Indenture referred to
below), for value received, hereby promises to pay to [_____
____________________________________________________], or registered assigns,
the principal sum of Dollars on the Stated Maturity specified above, and to pay
interest thereon from the Original Interest Accrual Date specified above or from
the most recent Interest Payment Date to which interest has been paid or duly
provided for, semi-annually in arrears on the Interest Payment Dates specified
above in each year, commencing [______________], and at Maturity, at the
Interest Rate per annum specified above, until the principal hereof is paid or
duly provided for. The interest so payable, and paid or duly provided for, on
any Interest Payment Date shall, as provided in such Indenture, be paid to the
Person in whose name this Security (or one or more Predecessor Securities) is
registered at the close of business on the Regular Record Date specified above
(whether or not a Business Day) next preceding such Interest Payment Date.
Notwithstanding the foregoing, interest payable at Maturity shall be paid to the
Person to whom principal shall be paid. Except as otherwise provided in said
Indenture, any such interest not so paid or duly provided for shall forthwith
cease to be payable to the Holder on such Regular Record Date and may either be
paid to the Person in whose name this Security (or one or more Predecessor
Securities) is registered at the close of business on a Special Record Date for
the payment of such Defaulted Interest to be fixed by the Trustee, notice of
which shall be given to Holders of Securities of this series not less than 15
days prior to such Special Record Date, or be paid in such other manner as
permitted by the Indenture.


                                      A-1
<PAGE>


         Payment of the principal of this Security and interest hereon at
Maturity shall be made upon presentation of this Security at the Corporate Trust
Office of U.S. Bank Trust National Association, in New York, New York or at such
other office or agency as may be designated for such purpose by the Company from
time to time. Payment of interest on this Security (other than interest at
Maturity) shall be made by check mailed to the address of the Person entitled
thereto as such address shall appear in the Security Register, except that if
such Person shall be a securities depositary, such payment may be made by such
other means in lieu of check as shall be agreed upon by the Company, the Trustee
and such Person. Payment of the principal of and interest on this Security, as
aforesaid, shall be made in such coin or currency of the United States of
America as at the time of payment shall be legal tender for the payment of
public and private debts.

         This Security is one of a duly authorized issue of securities of the
Company (herein called the "Securities"), issued and issuable in one or more
series under and equally secured by an Indenture, dated as of October 1, 1993
(such Indenture as originally executed and delivered and as supplemented or
amended from time to time thereafter, together with any constituent instruments
establishing the terms of particular Securities, being herein called the
"Indenture"), between the Company and U.S. Bank Trust National Association
(formerly First Trust of New York, National Association) as successor trustee
(herein called the "Trustee," which term includes any successor trustee under
the Indenture), to which Indenture and all indentures supplemental thereto
reference is hereby made for a description of the property mortgaged, pledged
and held in trust, the nature and extent of the security and the respective
rights, limitations of rights, duties and immunities of the Company, the Trustee
and the Holders of the Securities thereunder and of the terms and conditions
upon which the Securities are, and are to be, authenticated and delivered and
secured. The acceptance of this Security shall be deemed to constitute the
consent and agreement by the Holder hereof to all of the terms and provisions of
the Indenture. This Security is one of the series designated above.

         If any Interest Payment Date or the Stated Maturity shall not be a
Business Day (as hereinafter defined), payment of the amounts due on this
Security on such date may be made on the next succeeding Business Day; and, if
such payment is made or duly provided for on such Business Day, no interest
shall accrue on such amounts for the period from and after such Interest Payment
Date or Stated Maturity, as the case may be, to such Business Day.

         [This Security is not subject to redemption prior to the Stated
Maturity hereof.]

         [This Security is subject to redemption at any time on or after
___________, as a whole at any time or from time to time in part, at the option
of the Company, at a redemption price equal to [__________].

         [Notwithstanding the foregoing, the Company may not, prior to
__________, redeem this Security as contemplated above as a part of, or in
anticipation of, any refunding operation by the application, directly or
indirectly, of moneys borrowed having an effective interest cost to the Company
(calculated in accordance with generally accepted financial practice) less than
the effective interest cost to the Company (similarly calculated) of this
Security.]

         [Insert provisions, if any, for redemption pursuant to a sinking fund
or analogous provision or at the option of the holder.]

         [Notice of redemption shall be given by mail to the Holder of this
Security, not less than 30 days nor more than 60 days prior to the date fixed
for redemption, all as provided in the Indenture. As provided in the Indenture,
notice of redemption at the election of the Company as aforesaid may state that
such redemption shall be conditional upon the receipt by the Paying Agent or
Agents for this Security, on or prior to the date fixed for such redemption, of
money sufficient to pay the principal of and premium, if


                                      A-2
<PAGE>


any, and interest, on this Security; a notice of redemption so conditioned shall
be of no force or effect if such money is not so received and, in such event,
the Company shall not be required to redeem this Security].

         [In the event of redemption of this Security in part only, a new
Security or Securities of this series, of like tenor, for the unredeemed portion
hereof and otherwise having the same terms as this Security will be issued in
the name of the Holder hereof upon the cancellation hereof.]

         If an Event of Default shall occur and be continuing, the principal of
this Security may be declared due and payable in the manner and with the effect
provided in the Indenture.

         The Indenture permits, with certain exceptions as therein provided, the
Trustee to enter into one or more supplemental indentures for the purpose of
adding any provisions to, or changing in any manner or eliminating any of the
provisions of, the Indenture with the consent of the Holders of not less than a
majority in aggregate principal amount of the Securities of all series then
Outstanding under the Indenture, considered as one class; provided, however,
that if there shall be Securities of more than one series Outstanding under the
Indenture and if a proposed supplemental indenture shall directly affect the
rights of the Holders of Securities of one or more, but less than all, of such
series, then the consent only of the Holders of a majority in aggregate
principal amount of the Outstanding Securities of all series so directly
affected, considered as one class, shall be required; and provided, further,
that if the Securities of any series shall have been issued in more than one
Tranche and if the proposed supplemental indenture shall directly affect the
rights of the Holders of Securities of one or more, but less than all, of such
Tranches, then the consent only of the Holders of a majority in aggregate
principal amount of the Outstanding Securities of all Tranches so directly
affected, considered as one class, shall be required; and provided, further,
that the Indenture permits the Trustee to enter into one or more supplemental
indentures for limited purposes without the consent of any Holders of
Securities. The Indenture also contains provisions permitting the Holders of a
majority in principal amount of the Securities then Outstanding, on behalf of
the Holders of all Securities, to waive compliance by the Company with certain
provisions of the Indenture and certain past defaults under the Indenture and
their consequences. Any such consent or waiver by the Holder of this Security
shall be conclusive and binding upon such Holder and upon all future Holders of
this Security and of any Security issued upon the registration of transfer
hereof or in exchange therefor or in lieu hereof, whether or not notation of
such consent or waiver is made upon this Security.

         As provided in the Indenture and subject to certain limitations therein
set forth, this Security or any portion of the principal amount hereof will be
deemed to have been paid for all purposes of the Indenture and to be no longer
Outstanding thereunder, and, at the election of the Company, the Company's
entire indebtedness in respect thereof will be satisfied and discharged, if
there has been irrevocably deposited with the Trustee or any Paying Agent (other
than the Company), in trust, money in an amount which will be sufficient and/or
Eligible Obligations, the principal of and interest on which when due, without
regard to any reinvestment thereof, will provide moneys which, together with
moneys so deposited, will be sufficient, to pay when due the principal of and
interest on this Security when due.

         As provided in the Indenture and subject to certain limitations therein
set forth, the transfer of this Security is registrable in the Security
Register, upon surrender of this Security for registration of transfer at the
office of U.S. Bank Trust National Association, in New York, New York or such
other office or agency as may be designated by the Company from time to time,
duly endorsed by, or accompanied by a written instrument of transfer in form
satisfactory to the Company and the Security Registrar duly executed by, the
Holder hereof or his attorney duly authorized in writing, and thereupon one or
more new Securities of this series of authorized denominations and of like tenor
and aggregate principal amount, will be issued to the designated transferee or
transferees.


                                      A-3
<PAGE>


         The Securities of this series are issuable only as registered
Securities, without coupons, and in denominations of $1,000 and integral
multiples thereof. As provided in the Indenture and subject to certain
limitations therein set forth, Securities of this series are exchangeable for a
like aggregate principal amount of Securities of the same series, of any
authorized denominations, as requested by the Holder surrendering the same, and
of like tenor upon surrender of the Security or Securities to be exchanged at
the office of U.S. Bank Trust National Association, in New York, New York or
such other office or agency as may be designated by the Company from time to
time.

         [The Company shall not be required to execute or provide for the
registration of transfer of or the exchange of (a) Securities of this series
during a period of 15 day immediately preceding the date notice is given
identifying the serial numbers of the Securities of this series called for
redemption or (b) any Security so selected for redemption in whole or in part,
except the unredeemed portion of any Security being redeemed in part.]

         No service charge shall be made for any such registration of transfer
or exchange, but the Company may require payment of a sum sufficient to cover
any tax or other governmental charge payable in connection therewith.

         Prior to due presentment of this Security for registration of transfer,
the Company, the Trustee and any agent of the Company or the Trustee may treat
the Person in whose name this Security is registered as the absolute owner
hereof for all purposes, whether or not this Security be overdue, and neither
the Company, the Trustee nor any such agent shall be affected by notice to the
contrary.

         The Indenture and the Securities shall be governed by and construed in
accordance with the laws of the State of New York.

         As used herein "Business Day" means any day, other than a Saturday or
Sunday, which is not a day on which banking institutions or trust companies in
The City of New York, New York or other city in which is located any office or
agency maintained for the payment of principal or interest on this Security, are
authorized or required by law, regulation or executive order to remain closed.
All other terms used in this Security which are defined in the Indenture shall
have the meanings assigned to them in the Indenture.

         As provided in the Indenture, no recourse shall be had for the payment
of the principal of[, premium, if any] or interest on any Securities, or any
part thereof, or for any claim based thereon or otherwise in respect thereof, or
of the indebtedness represented thereby, or upon any obligation, covenant or
agreement under the Indenture, against, and no personal liability whatsoever
shall attach to, or be incurred by, any incorporator, shareholder, officer or
director, as such, past, present or future of the Company or of any predecessor
or successor corporation (either directly or through the Company or a
predecessor or successor corporation), whether by virtue of any constitutional
provision, statute or rule of law, or by the enforcement of any assessment or
penalty or otherwise; it being expressly agreed and understood that the
Indenture and all the Securities are solely corporate obligations and that any
such personal liability is hereby expressly waived and released as a condition
of, and as part of the consideration for, the execution of the Indenture and the
issuance of the Securities.

         Unless the certificate of authentication hereon has been executed by
the Trustee or an Authenticating Agent by manual signature, this Security shall
not be entitled to any benefit under the Indenture or be valid or obligatory for
any purpose.


                                      A-4
<PAGE>
IN WITNESS WHEREOF, the Company has caused this instrument to be duly executed
and its corporate seal to be hereunto affixed and attested.

                                      PUBLIC SERVICE COMPANY OF COLORADO

                                      By:
                                         ---------------------------------------
                                               [Title]




Attest:
       ----------------------------
              [Title]


                          CERTIFICATE OF AUTHENTICATION

         This is one of the Securities of the series designated therein referred
to in the within-mentioned Indenture.

Dated:
      -----------------------------


            U.S. BANK TRUST              OR             U.S. BANK TRUST
         NATIONAL ASSOCIATION,                       NATIONAL ASSOCIATION,
              as Trustee                                  as Trustee


By:                                             By:
   --------------------------------                -----------------------------
         Authorized Officer                         as Authenticating Agent


                                                By:
                                                   -----------------------------
                                                     Authorized Officer





         UNLESS THIS CERTIFICATE IS PRESENTED BY AN AUTHORIZED REPRESENTATIVE OF
THE DEPOSITORY TRUST COMPANY, A NEW YORK CORPORATION ("DTC"), TO THE COMPANY OR
ITS AGENT FOR REGISTRATION OF TRANSFER, EXCHANGE, OR PAYMENT, AND ANY
CERTIFICATE ISSUED IS REGISTERED IN THE NAME OF CEDE & CO. OR IN SUCH OTHER NAME
AS IS REQUESTED BY AN AUTHORIZED REPRESENTATIVE OF DTC (AND ANY PAYMENT IS MADE
TO CEDE & CO. OR TO SUCH OTHER ENTITY AS IS REQUESTED BY AN AUTHORIZED
REPRESENTATIVE OF DTC), ANY TRANSFER, PLEDGE, OR OTHER USE HEREOF FOR VALUE OR
OTHERWISE BY OR TO ANY PERSON IS WRONGFUL INASMUCH AS THE REGISTERED OWNER
HEREOF, CEDE & CO., HAS AN INTEREST HEREIN.

         THIS SECURITY MAY NOT BE TRANSFERRED OR EXCHANGED, NOR MAY ANY
PURPORTED TRANSFER BE REGISTERED, EXCEPT (i) THIS SECURITY MAY BE TRANSFERRED IN
WHOLE, AND APPROPRIATE REGISTRATION OF TRANSFER EFFECTED, IF SUCH TRANSFER IS BY
CEDE & CO., AS NOMINEE FOR THE DEPOSITORY TRUST COMPANY (THE "DEPOSITARY"), TO
THE DEPOSITARY, OR BY THE DEPOSITARY TO ANOTHER NOMINEE THEREOF, OR BY ANY
NOMINEE OF THE DEPOSITARY TO ANY OTHER NOMINEE THEREOF, OR BY THE DEPOSITARY OR
ANY NOMINEE THEREOF TO ANY SUCCESSOR SECURITIES DEPOSITARY OR ANY NOMINEE
THEREOF; AND (II) THIS SECURITY MAY BE EXCHANGED FOR DEFINITIVE SECURITIES
REGISTERED IN THE RESPECTIVE NAMES OF THE BENEFICIAL HOLDERS HEREOF, AND
THEREAFTER SHALL BE TRANSFERABLE WITHOUT RESTRICTIONS IF: (A) THE DEPOSITARY, OR
ANY SUCCESSOR SECURITIES DEPOSITARY, SHALL HAVE NOTIFIED THE COMPANY AND THE
TRUSTEE THAT IT IS UNWILLING OR UNABLE TO CONTINUE TO ACT AS SECURITIES
DEPOSITARY WITH RESPECT TO THE SECURITIES AND THE TRUSTEE SHALL NOT HAVE BEEN
NOTIFIED BY THE COMPANY WITHIN NINETY (90) DAYS OF THE IDENTITY OF A SUCCESSOR
SECURITIES DEPOSITARY WITH RESPECT TO THE SECURITIES; (B) THE COMPANY SHALL HAVE
DELIVERED TO THE TRUSTEE A


                                      A-5
<PAGE>


COMPANY ORDER TO THE EFFECT THAT THE SECURITIES SHALL BE SO EXCHANGEABLE ON AND
AFTER A DATE SPECIFIED THEREIN; OR (C)(1) AN EVENT OF DEFAULT SHALL HAVE
OCCURRED AND BE CONTINUING, (2) THE TRUSTEE SHALL HAVE GIVEN NOTICE OF SUCH
EVENT OF DEFAULT PURSUANT TO SECTION 1102 OF THE INDENTURE AND (3) THERE SHALL
HAVE BEEN DELIVERED TO THE COMPANY AND THE TRUSTEE AN OPINION OF COUNSEL TO THE
EFFECT THAT THE INTERESTS OF THE BENEFICIAL OWNERS OF THE SECURITIES IN RESPECT
THEREOF WILL BE MATERIALLY IMPAIRED UNLESS SUCH OWNERS BECOME HOLDERS OF
DEFINITIVE SECURITIES.

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


                                      A-6
<PAGE>


         FOR VALUE RECEIVED the undersigned hereby sells, assigns and transfers
unto


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

     [please insert social security or other identifying number of assignee]


--------------------------------------------------------------------------------
            [please print or typewrite name and address of assignee]


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

the within Security of PUBLIC SERVICE COMPANY OF COLORADO and does hereby
irrevocably constitute and appoint __________________________________ ,
Attorney, to transfer said Security on the books of the within-mentioned
Company, with full power of substitution in the premises.

Dated:
      ---------------------------


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

         Notice: The signature to this assignment must correspond with the name
         as written upon the face of the Security in every particular without
         alteration or enlargement or any change whatsoever.


                                      A-7
<PAGE>

                            OPTION TO ELECT REPAYMENT

         The undersigned hereby irrevocably request(s) and instruct(s) the
Company to repay this Note (or portion hereof specified below) pursuant to its
terms at a price equal to 100% of the principal amount to be repaid, together
with unpaid interest accrued hereon to the Repayment Date, to the undersigned,
at _____________________________________________________________________________
(Please print or typewrite name and address of the undersigned).

         For this Note to be repaid, the Trustee must receive at its Corporate
Trust Office in New York, New York not more than 60 nor less than 30 calendar
days prior to the Repayment Date, this Note with this "Option to Elect
Repayment" form duly completed.

         If less than the entire principal amount of this Note is to be repaid,
specify the portion hereof (which shall be increments of $1,000 (provided that
any remaining principal amount hereof shall be at least $100,000)) which the
Holder elects to have repaid and specify the denomination or denominations
(which shall be a minimum of $100,000) of the Notes to be issued to the Holder
for the portion of this Note not being repaid (in the absence of any such
specification, one such Note will be issued for the portion not being repaid).

         Principal Amount to be Repaid:  $
                                          -------------------------------

         Date:
               -----------------------------

         NOTICE: The signature(s) on this Option to Elect Repayment must
correspond with the name(s) as written upon the face of this Note in every
particular, without alteration or enlargement or any change whatsoever.


                                      A-8
<PAGE>


                                                                      SCHEDULE A


                             SUPPLEMENTAL INDENTURES

<Table>
<Caption>
        DATE OF                                                                                  PRINCIPAL
      SUPPLEMENTAL                                                       PRINCIPAL                 AMOUNT
       INDENTURE                      SERIES OF BONDS                  AMOUNT ISSUED            OUTSTANDING
      ------------                    ---------------                  -------------            ------------
<S>                                <C>                                 <C>                      <C>
November 1, 1993                        Series No. 1                    $134,500,000            $134,500,000
January 1, 1994                    Series No. 2 due 2001                $102,667,000                None
                                            and
                                   Series No. 2 due 2024                $110,000,000            $110,000,000
September 2, 1994                           None                            None                    None
(Appointment of
Successor Trustee)
May 1, 1996                             Series No. 3                    $125,000,000            $125,000,000
November 1, 1996                        Series No. 4                    $250,000,000            $175,000,000
February 1, 1997                        Series No. 5                    $150,000,000                None
April 1, 1998                           Series No. 6                    $250,000,000            $250,000,000
August 15, 2002                         Series No. 7                    $ 48,750,000            $ 48,750,000
September 1, 2002                       Series No. 8                    $600,000,000            $600,000,000
September 15, 2002                      Series No. 9                    $530,000,000            $530,000,000
March 1, 2003                          Series No. 11                    $250,000,000            $250,000,000
</Table>


                                      A-9