<PAGE>

                                                                    EXHIBIT 4(B)


                             TNP ENTERPRISES, INC.,
                                   as Issuer,

                                      and

                             THE BANK OF NEW YORK,
                                   as Trustee

                              ___________________

                          FIRST SUPPLEMENTAL INDENTURE

                           Dated as of April 7, 2000

                                       to

                                   INDENTURE

                           Dated as of April 7, 2000

                                    between

                             ST ACQUISITION CORP.,
                                   as Issuer,

                                      and

                             THE BANK OF NEW YORK,
                                   as Trustee
                              ___________________

                                  $275,000,000

                   10.25% Senior Subordinated Notes due 2010
<PAGE>

          FIRST SUPPLEMENTAL INDENTURE, dated as of April 7, 2000, by and
between TNP ENTERPRISES, INC., a Texas corporation, as issuer (the "Company"),
and The Bank of New York, as trustee (the "Trustee").

          WHEREAS, ST Acquisition Corp., a Texas corporation ("Acquisition"),
has heretofore executed and delivered to the Trustee an Indenture dated as of
April 7, 2000 (the "Indenture"), providing for the issuance of its 10.25% Senior
Subordinated Notes due 2010 in the principal amount of $275,000,000 (the
"Securities"); and

          WHEREAS, Acquisition has merged with and into the Company and, in
connection herewith, the Company has assumed by operation of law all of
Acquisition's debts, liabilities, duties and obligations, including
Acquisition's obligations in respect of the Securities and under the Indenture;
and

          WHEREAS, the Company desires by this First Supplemental Indenture,
pursuant to and as contemplated by Sections 5.01 and 8.01 of the Indenture, to
expressly assume the covenants, agreements and undertakings of Acquisition in
the Indenture and under the Securities; and

          WHEREAS, the execution and delivery of this First Supplemental
Indenture and the notes evidencing the Securities, substantially in the form
attached hereto as Exhibit A, have been authorized by a resolution of the Board
of Directors of the Company; and

          WHEREAS, the Company authorizes the Trustee to cancel the 10.25 %
Senior Subordinated Notes due 2010 of Acquisition and to authenticate
$275,000,000 principal amount of 10.25% Senior Subordinated Notes due 2010 of
the Company in replacement thereof; and

          WHEREAS, all conditions and requirements necessary to make each of
this First Supplemental Indenture and the Securities a valid, binding and legal
instrument in accordance with its terms upon the Company and the Trustee have
been performed and fulfilled by the applicable parties hereto and the execution
and delivery thereof have been in all respects duly authorized by the applicable
parties hereto.

          NOW, THEREFORE, in consideration of the above premises, each party
agrees, for the benefit of the others and for the equal and ratable benefit of
the Holders of the Securities, as follows:
<PAGE>

                                     -2-

                                  ARTICLE ONE

                           ASSUMPTION OF OBLIGATIONS

          SECTION 1.01.  Assumption of Obligations of Acquisition.
                         ----------------------------------------

          (a)  The Company hereby expressly and unconditionally assumes each and
every covenant, agreement and undertaking of Acquisition in the Indenture as if
the Company had been the original issuer of the Securities, and also hereby
expressly and unconditionally assumes each and every covenant, agreement and
undertaking in each Security outstanding on the date of this First Supplemental
Indenture.

          (b)  Promptly following the execution and delivery of this First
Supplemental Indenture, the Trustee shall, upon the written order of the Company
in the form of an Officers' Certificate of the Company, authenticate and deliver
Securities substantially in the form of Exhibit A hereto in replacement of the
outstanding Securities.

          SECTION 1.02.  Exchange of Outstanding Securities; Exhibits.
                         --------------------------------------------

          Exhibits A, B, C, D, E and F of the Indenture are hereby deleted and
replaced in their entirety by Exhibits A, B, C, D, E and F, respectively,
hereto.

                                  ARTICLE TWO

                           MISCELLANEOUS PROVISIONS

          SECTION 2.01.  Terms Defined.
                         -------------

          For all purposes of this First Supplemental Indenture, except as
otherwise defined or unless the context otherwise requires, terms used in
capitalized form in this First Supplemental Indenture and defined in the
Indenture have the meanings specified in the Indenture.

          SECTION 2.02.  Indenture.
                         ---------

          Except as amended hereby, the Indenture and the Securities are in all
respects ratified and confirmed and all the terms shall remain in full force and
effect.
<PAGE>

                                      -3-

          SECTION 2.03.  Governing Law.
                         -------------

          THIS FIRST SUPPLEMENTAL INDENTURE SHALL BE GOVERNED BY AND CONSTRUED
IN ACCORDANCE WITH THE LAWS OF THE STATE OF NEW YORK, AS APPLIED TO CONTRACTS
MADE AND PERFORMED WITHIN THE STATE OF NEW YORK, WITHOUT REGARD TO PRINCIPLES OF
CONFLICT OF LAWS.

          SECTION 2.04.  Successors.
                         ----------

          All agreements of the Company in this First Supplemental Indenture and
the Securities shall bind its successors.  All agreements of the Trustee in this
First Supplemental Indenture shall bind its successors.

          SECTION 2.05.  Duplicate Originals.
                         -------------------

          The parties may sign any number of copies of this First Supplemental
Indenture.  Each signed copy shall be an original, but all of them together
shall represent the same agreement.

          SECTION 2.06.  Trustee Disclaimer.
                         ------------------

          The Trustee accepts the amendment of the Indenture effected by this
First Supplemental Indenture and agrees to execute the trust created by the
Indenture as hereby amended, but only upon the terms and conditions set forth in
the Indenture, including the terms and provisions defining and limiting the
liabilities and responsibilities of the Trustee, which terms and provisions
shall in like manner define and limit its liabilities and responsibilities in
the performance of the trust created by the Indenture as hereby amended and,
without limiting the generality of the foregoing, the Trustee shall not be
responsible in any manner whatsoever for or with respect to any of the recitals
or statements contained herein, all of which recitals or statements are made
solely by the Company, or for or with respect to (i) the validity of the terms
of this First Supplemental Indenture or any of the terms or provisions hereof,
(ii) the proper authorization hereof by the Company by corporate action or
otherwise, (iii) the due execution hereof by the Company or (iv) the
consequences (direct or indirect and whether deliberate or inadvertent) of any
amendment herein provided for, and the Trustee makes no representation with
respect to any such matters.


                           [Signature Page Follows]
<PAGE>

                                  SIGNATURES

          IN WITNESS WHEREOF, the parties hereto have caused this First
Supplemental Indenture to be duly executed as of the date first written above.

                                       TNP ENTERPRISES, INC.,
                                        as issuer

                                       By:  ____________________________
                                             Name:
                                             Title:

                                       THE BANK OF NEW YORK,
                                         as trustee

                                       By:  ____________________________
                                             Name:
                                             Title:
<PAGE>

                                                                    EXHIBIT A
                                                                    ---------

                                                            CUSIP No.:

                             TNP ENTERPRISES, INC.

                   10.25% SENIOR SUBORDINATED NOTE DUE 2010

No.                                                                   $

          TNP ENTERPRISES, INC., a Texas corporation (the "Company," which term
includes any successor entity), for value received promises to pay to
or registered assigns, the principal sum of [                           ]
DOLLARS on April 1, 2010.

          Interest Payment Dates:  April 1 and October 1, commencing October 1,
2000.

          Record Dates:  March 15 and September 15.

          Reference is made to the further provisions of this Note contained
herein and the Indenture (as defined), which will for all purposes have the same
effect as if set forth at this place.

          IN WITNESS WHEREOF, the Company has caused this Note to be signed
manually by its duly authorized officers and a facsimile of its corporate seal
to be affixed hereto or imprinted hereon.

                                    TNP ENTERPRISES, INC.



                                    By:  ______________________
                                         Name:
                                         Title:


                                    By:  ______________________
                                         Name:
                                         Title:

                                      A-1
<PAGE>

Certificate of Authentication

          This is one of the 10.25% Senior Subordinated Notes due 2010 referred
to in the within-mentioned Indenture.

                                      The Bank of New York,
                                        as Trustee


                                      By: ____________________________
                                          Authorized Signatory

Date of Authentication:

                                      A-2
<PAGE>

                             (REVERSE OF SECURITY)

                   10.25% SENIOR SUBORDINATED NOTE DUE 2010

          1.   Interest.  TNP ENTERPRISES, INC., a Texas corporation (the
               --------
"Company"), promises to pay interest on the principal amount of this Note at the
--------
rate per annum shown above.  Interest on the Notes will accrue from the most
recent date on which interest has been paid or, if no interest has been paid,
from the date of the original issuance of the Notes.  The Company will pay
interest semi-annually in arrears on each Interest Payment Date, commencing
October 1, 2000.  Interest will be computed on the basis of a 360-day year of
twelve 30-day months.

          The Company shall pay interest on overdue principal and on overdue
installments of interest (without regard to any applicable grace periods) to the
extent lawful from time to time on demand at the rate borne by the Notes.

          2.  Method of Payment. The Company shall pay interest on the Notes
              -----------------
(except defaulted interest) to the Persons who are the registered Holders at the
close of business on March 15 or September 15 immediately preceding the Interest
Payment Date (whether or not such day is a Business Day) even if the Notes are
cancelled on registration of transfer or registration of exchange after such
Record Date. Holders must surrender Notes to a Paying Agent to collect principal
payments. Payments of principal and premium, if any, will be made (on
presentation of such Notes if in certificated form) in money of the United
States that at the time of payment is legal tender for payment of public and
private debts; provided, however, that the Company may pay principal, premium,
if any, and interest by check payable in such money. The Company may deliver any
such interest payment to the Paying Agent or to a Holder at the Holder's
registered address.

          3.  Paying Agent and Registrar. Initially, The Bank of New York, a New
              -------------------------
York banking corporation (the "Trustee"), will act as Paying Agent and
                               -------
Registrar. The Company may change any Paying Agent, Registrar or co-Registrar
without notice to the Holders. Neither the Company nor any of its Subsidiaries
or Affiliates may act as Paying Agent but may act as Registrar or co-Registrar.

          4.  Indenture. The Company issued this Note under an Indenture, dated
              ---------
as of April 7, 2000 (the "Indenture"), by and between the Company and the
                          ---------
Trustee. This Note is one of a duly authorized issue of Initial Notes of the
Company designated as its 10.25% Senior Subordinated Notes due 2010 (the
"Notes"). The Notes are limited in aggregate principal amount to $275,000,000.
 -----
The Notes include the Initial Notes and the Exchange Notes (as defined below)
issued in exchange for the Initial Notes pursuant to the Indenture. The Initial
Notes and the Exchange Notes are treated as a single class of securities under
the Indenture. Capitalized terms herein are used as defined in the Indenture
unless otherwise defined

                                      A-3
<PAGE>

herein. The terms of the Notes include those stated in the Indenture and those
made part of the Indenture by reference to the Trust Indenture Act of 1939 (15
U.S. Code Sections 77aaa-77bbbb) (the "TIA"), as in effect on the date of the
Indenture. Notwithstanding anything to the contrary herein, the Notes are
subject to all such terms, and Holders are referred to the Indenture and the TIA
for a statement of them. The Notes are general unsecured obligations of the
Company.

          5.   Subordination. The Notes are unsecured obligations of the Company
               -------------
and subordinated in right of payment, in the manner and to the extent set forth
in the Indenture, to the prior payment in full in cash of all Senior
Indebtedness of the Company, whether outstanding on the date of the Indenture or
thereafter created, incurred, assumed or guaranteed. Each Holder by his
acceptance hereof agrees to be bound by such provisions and authorizes and
expressly directs the Trustee, on his behalf, to take such action as may be
necessary or appropriate to effectuate the subordination provided for in the
Indenture and appoints the Trustee his attorney-in-fact for such purposes.

          6.   Redemption.
               ----------

          (a)  Optional Redemption. The Company may redeem the Notes, at its
               -------------------
option, in whole at any time or in part from time to time, on and after April 1,
2005 at the following Redemption Prices (expressed as percentages of the
principal amount thereof) if redeemed during the twelve-month period commencing
on April 1 of the year set forth below, plus, in each case, accrued and unpaid
interest thereon, if any, to the date of redemption.


          Year                                      Percentage
          ----                                      ----------

          2005....................................  105.125%
          2006....................................  103.417%
          2007....................................  101.708%
          2008 and thereafter.....................  100.000%


          (b)  Optional Redemption Upon Equity Offerings.  Notwithstanding the
               -----------------------------------------
foregoing, the Company may redeem in the aggregate up to 35% of the original
principal amount of Notes at any time and from time to time prior to April 1,
2003 at a Redemption Price equal to 110.25% of the aggregate principal amount so
redeemed, plus accrued and unpaid interest, if any, to the Redemption Date out
of the Net Proceeds of one or more Public Equity Offerings; provided that

          (1)  at least 65% of the principal amount of Notes originally issued
remains outstanding immediately after the occurrence of any such redemption and

          (2)  any such redemption occurs within 90 days following the closing
of any such Public Equity Offering.

                                      A-4
<PAGE>

          7.   Notice of Redemption. Notice of redemption under paragraphs 6(a)
               --------------------
and (b) of this Note will be mailed at least 30 days but not more than 60 days
before the Redemption Date to each Holder to be redeemed at such Holder's
registered address.

          Except as set forth in the Indenture, if monies for the redemption of
the Notes called for redemption shall have been deposited with the Paying Agent
for redemption on such Redemption Date, then, unless the Company defaults in the
payment of such Redemption Price plus accrued interest, if any, the Notes called
for redemption will cease to bear interest from and after such Redemption Date
and the only right of the Holders of such Notes will be to receive payment of
the Redemption Price plus accrued interest, if any.

          8.   Offers to Purchase.  The Indenture provides that, after certain
               ------------------
Asset Sales (as defined in the Indenture) and upon the occurrence of a Change of
Control (as defined in the Indenture), and subject to further limitations
contained therein, the Company will make an offer to purchase certain amounts of
the Notes in accordance with the procedures set forth in the Indenture.

          9.   Registration Rights.  Pursuant to the Registration Rights
               -------------------
Agreement by and among the Company and the Initial Purchasers, the Company will
be obligated to consummate an exchange offer pursuant to which the Holder of
this Note shall have the right to exchange this Note for the Company's Series B
10.25% Senior Subordinated Notes due 2010 (the "Exchange Notes"), which have
                                                --------------
been registered under the Securities Act, in like principal amount and having
terms identical in all material respects to the Initial Notes.  The Holders of
the Initial Notes shall be entitled to receive certain Additional Interest
payments in the event such exchange offer is not consummated and upon certain
other conditions, all pursuant to and in accordance with the terms of the
Registration Rights Agreement.

          10.  Denominations; Transfer; Exchange.  The Notes are in registered
               ---------------------------------
form, without coupons, in denominations of $1,000 and integral multiples
thereof.  A Holder shall register the transfer or exchange of Notes in
accordance with the Indenture.  The Registrar may require a Holder, among other
things, to furnish appropriate endorsements and transfer documents and to pay
certain transfer taxes or similar governmental charges payable in connection
therewith as permitted by the Indenture.  The Registrar need not register the
transfer of or exchange of any Notes or portions thereof selected for
redemption.

          11.  Persons Deemed Owners.  The registered holder of a Note shall be
               ---------------------
treated as the owner of it for all purposes.

          12.  Unclaimed Money.  If money for the payment of principal or
               ---------------
interest remains unclaimed for two years, the Trustee and the Paying Agent will
pay the money back to the Company upon written request.  After that, Holders
entitled to money must look to the Company for payment as general creditors
unless an "abandoned property" law designates another person.

                                 A-5
<PAGE>

          13.  Legal Defeasance and Covenant Defeasance.  If the Company at any
               ----------------------------------------
time deposits with the Trustee U.S. legal tender or U.S. Government Obligations
sufficient to pay the principal of and interest on the Notes to redemption or
maturity and complies with the other provisions of the Indenture relating to
defeasance, the Company will be discharged from certain provisions of the
Indenture and the Notes (including certain covenants, but excluding its
obligation to pay the principal of and interest on the Notes).

          14.  Amendments, Supplements, and Waivers.  Subject to certain
               ------------------------------------
exceptions, the Indenture or the Notes may be amended or supplemented with the
written consent of the Holders of at least a majority in aggregate principal
amount of the Notes  then outstanding, and any existing Default or Event of
Default or noncompliance with any provision may be waived with the written
consent of the Holders of a majority in aggregate principal amount of the Notes
then outstanding.  Without notice to or consent of any Holder, the parties
thereto may amend or supplement the Indenture or the Notes to, among other
things, cure any ambiguity, defect or inconsistency, provide for uncertificated
Notes in addition to or in place of certificated Notes or make any other change
that does not adversely affect in any material respect the rights of any Holder.

          15.  Restrictive Covenants.  The Indenture imposes certain limitations
               ---------------------
on the ability of the Company and its Subsidiaries to, among other things, incur
additional Indebtedness, make payments in respect of its Capital Stock, enter
into transactions with Affiliates, create dividend or other payment restrictions
affecting Restricted Subsidiaries, sell assets, create liens, issue capital
stock, enter into sale and lease-back transactions, make certain Investments,
merge or consolidate with any other Person, or sell, assign, transfer, lease,
convey or otherwise dispose of all or substantially all of its assets.  Such
limitations are subject to a number of important qualifications and exceptions.
The Company must quarterly report to the Trustee on compliance with such
limitations.

          16.  Successor Entity.  When a successor entity assumes, in accordance
               ----------------
with the Indenture, all the obligations of its predecessor under the Notes and
the Indenture, and immediately before and thereafter no Default or Event of
Default exists and certain other conditions are satisfied, the predecessor
entity will be released from those obligations.

          17.  Defaults and Remedies.  Events of Default are set forth in the
               ---------------------
Indenture.  If an Event of Default (other than an Event of Default pursuant to
Section 6.01(f) or (g) of the Indenture) shall have occurred and be continuing,
then the Trustee or the Holders of not less than 25% in aggregate principal
amount of the Notes then outstanding, may declare to be immediately due and
payable the entire principal amount of all the Notes then outstanding plus
accrued interest to the date of acceleration; provided, however, that after such
acceleration but before a judgment or decree based on such acceleration is
obtained by the Trustee, the Holders of a majority in aggregate principal amount
of the outstanding Notes may rescind and annul such acceleration and its
consequences if all existing Events of Default, other than the nonpayment of
principal, premium, if any, or interest that has become due solely because of

                                      A-6
<PAGE>

the acceleration, have been cured or waived. No such rescission shall affect any
subsequent Default or impair any right consequent thereto.  In case an Event of
Default specified in Section 6.01(f) or (g) of the Indenture occurs, such
principal amount, together with premium, if any, and interest with respect to
all of the Notes, shall be due and payable immediately without any declaration
or other act on the part of the Trustee or the Holders.

          18.  Trustee Dealings with Company.  The Trustee under the Indenture,
               -----------------------------
in its individual or any other capacity, may make loans to, accept deposits
from, and perform services for the Company, and may otherwise deal  with the
Company, its Subsidiaries or their respective Affiliates as if it were not the
Trustee.

          19.  No Recourse Against Others.  As more fully described in the
               --------------------------
Indenture, no director, officer, employee, stockholder or incorporator, as such,
of the Company shall have any liability for any obligation of the Company under
the Notes or the Indenture or for any claim based on, in respect of or by reason
of such obligations or their creation.  Each Holder by accepting a Note waives
and releases all such liability.  Such waiver and release are part of the
consideration for the issuance of the Notes.

          20.  Authentication.  This Note shall not be valid until the Trustee
               --------------
or Authenticating Agent manually signs the certificate of authentication on this
Note.

          21.  Governing Law.  THIS NOTE SHALL BE GOVERNED BY AND CONSTRUED IN
               -------------
ACCORDANCE WITH THE LAWS OF THE STATE OF NEW YORK, AS APPLIED TO CONTRACTS MADE
AND PERFORMED WITHIN THE STATE OF NEW YORK, WITHOUT REGARD TO PRINCIPLES OF
CONFLICTS OF LAW.  EACH OF THE PARTIES TO THE INDENTURE HAS AGREED TO SUBMIT TO
THE JURISDICTION OF THE COURTS OF THE STATE OF NEW YORK IN ANY ACTION OR
PROCEEDING ARISING OUT OF OR RELATING TO THIS NOTE.

          22.  Abbreviations and Defined Terms.  Customary abbreviations may be
               -------------------------------
used in the name of a Holder or an assignee, such as:  TEN COM (= tenants in
common), TEN ENT (= tenants by the entireties), JT TEN (= joint tenants with
right of survivorship and not as tenants in common), CUST (= Custodian), and
U/G/M/A (= Uniform Gifts to Minors Act).

          23.  CUSIP Numbers.  Pursuant to a recommendation promulgated by the
               -------------
Committee on Uniform Security Identification Procedures, the Company has caused
CUSIP numbers to be printed on the Notes as a convenience to the Holders.  No
representation is made as to the accuracy of such numbers as printed on the
Notes and reliance may be placed only on the other identification numbers
printed hereon.

                                      A-7
<PAGE>

          24.  Indenture.  Each Holder, by accepting a Note, agrees to be bound
               ---------
by all of the terms and provisions of the Indenture, as the same may be amended
from time to time.

          The Company will furnish to any Holder upon written request and
without charge a copy of the Indenture, which has the text of this Note in
larger type.  Requests may be made to:  TNP Enterprises, Inc., c/o Laurel Hill
Capital Partners LLC, 2 Robbins Lane, Suite 201, Jericho, New York 11753,
Attention:  Kathleen Marion.

                                      A-8
<PAGE>

                                ASSIGNMENT FORM

          If you the Holder want to assign this Note, fill in the form below and
have your signature guaranteed:

I or we assign and transfer this Note to:

_______________________________________________________________________________

_______________________________________________________________________________

_______________________________________________________________________________
                 (Print or type name, address and zip code and
                 social security or tax ID number of assignee)

and irrevocably appoint _______________________________________________________,
agent to transfer this Note on the books of the Company.  The agent may
substitute another to act for him.

Date:____________________  Signed: ____________________________________________
                                    (Sign exactly as your name appears on the
                                    other side of this Note)

Medallion Guarantee: _____________________________

                                      A-9
<PAGE>

                     [OPTION OF HOLDER TO ELECT PURCHASE]

          If you want to elect to have this Note purchased by the Company
pursuant to Section 4.13 or Section 4.16 of the Indenture, check the appropriate
box:

                                Section 4.13 [_]

                                Section 4.16 [_]

          If you want to elect to have only part of this Note purchased by the
Company pursuant to Section 4.13 or Section 4.16 of the Indenture, state the
amount you elect to have purchased:

$ _____________

Date:__________________
                             ________________________________________________
                             NOTICE:  The signature on this assignment must
                             correspond with the name as it appears upon the
                             face of the within Note in every particular without
                             alteration or enlargement or any change whatsoever
                             and be guaranteed by the endorser's bank or broker.

Medallion Guarantee:_____________________

                                     A-10
<PAGE>

                                                                       EXHIBIT B
                                                                       ---------
                                                               CUSIP No.:

                             TNP ENTERPRISES, INC.

               SERIES B 10.25% SENIOR SUBORDINATED NOTE DUE 2010

No.                                                                       $

          TNP ENTERPRISES, INC., a Texas corporation (the "Company," which term
includes any successor entity), for value received promises to pay to        or
registered assigns, the principal sum of [           ] DOLLARS on April 1, 2010.

          Interest Payment Dates:  April 1 and October 1, commencing October 1,
2000.

          Record Dates:  March 15 and September 15.

          Reference is made to the further provisions of this Note contained
herein and the Indenture (as defined), which will for all purposes have the same
effect as if set forth at this place.

          IN WITNESS WHEREOF, the Company has caused this Note to be signed
manually by its duly authorized officers and a facsimile of its corporate seal
to be affixed hereto or imprinted hereon.

                                    TNP ENTERPRISES, INC.

                                    By:  _______________________
                                         Name:
                                         Title:


                                    By:  _______________________
                                         Name:
                                         Title:

                                      B-1
<PAGE>

Certificate of Authentication

          This is one of the Series B 10.25% Senior Subordinated Notes due 2010
referred to in the within-mentioned Indenture.

                                    The Bank of New York,
                                     as Trustee

                                    By:  __________________________
                                         Authorized Signatory

Date of Authentication:

                                      B-2
<PAGE>

                             (REVERSE OF SECURITY)

                   10.25% SENIOR SUBORDINATED NOTE DUE 2010

          1.   Interest.  TNP ENTERPRISES, INC., a Texas corporation (the
               --------
"Company"), promises to pay interest on the principal amount of this Note at the
 -------
rate per annum shown above.  Interest on the Notes will accrue from the most
recent date on which interest has been paid or, if no interest has been paid,
from the date of the original issuance of the Notes.  The Company will pay
interest semi-annually in arrears on each Interest Payment Date, commencing
October 1, 2000.  Interest will be computed on the basis of a 360-day year of
twelve 30-day months.

          The Company shall pay interest on overdue principal and on overdue
installments of interest (without regard to any applicable grace periods) to the
extent lawful from time to time on demand at the rate borne by the Notes.

          2.   Method of Payment. The Company shall pay interest on the Notes
               -----------------
(except defaulted interest) to the Persons who are the registered Holders at the
close of business on March 15 or September 15 immediately preceding the Interest
Payment Date (whether or not such day is a Business Day) even if the Notes are
cancelled on registration of transfer or registration of exchange after such
Record Date. Holders must surrender Notes to a Paying Agent to collect principal
payments. Payments of principal and premium, if any, will be made (on
presentation of such Notes if in certificated form) in money of the United
States that at the time of payment is legal tender for payment of public and
private debts; provided, however, that the Company may pay principal, premium,
if any, and interest by check payable in such money. The Company may deliver any
such interest payment to the Paying Agent or to a Holder at the Holder's
registered address.

          3.   Paying Agent and Registrar. Initially, The Bank of New York, a
               --------------------------
New York banking corporation (the "Trustee"), will act as Paying Agent and
                                   -------
Registrar. The Company may change any Paying Agent, Registrar or co-Registrar
without notice to the Holders. Neither the Company nor any of its Subsidiaries
or Affiliates may act as Paying Agent but may act as Registrar or co-Registrar.

          4.   Indenture. The Company issued this Note under an Indenture, dated
               ---------
as of April 7, 2000 (the "Indenture"), by and between the Company and the
                          ---------
Trustee. This Note is one of a duly authorized issue of Notes of the Company
designated as its Series B 10.25% Senior Subordinated Notes due 2010 (the
"Exchange Notes") issued in exchange for the initial 10.25% Senior Subordinated
 --------------
Notes due 2010 (the "Initial Notes" and, together with the Exchange Notes, the
                     -------------
"Notes"). The Notes are limited in aggregate principal amount to $275,000,000.
 -----
Capitalized terms herein are used as defined in the Indenture unless otherwise
defined herein. The terms of the Notes include those stated in the Indenture and
those made

                                      B-3
<PAGE>

part of the Indenture by reference to the Trust Indenture Act of 1939 (15 U.S.
Code Sections 77aaa-77bbbb) (the "TIA"), as in effect on the date of the
                                  ---
Indenture. Notwithstanding anything to the contrary herein, the Notes are
subject to all such terms, and Holders are referred to the Indenture and the TIA
for a statement of them. The Notes are general unsecured obligations of the
Company.

          5.  Subordination. The Notes are unsecured obligations of the Company
              -------------
and subordinated in right of payment, in the manner and to the extent set forth
in the Indenture, to the prior payment in full in cash of all Senior
Indebtedness of the Company, whether outstanding on the date of the Indenture or
thereafter created, incurred, assumed or guaranteed. Each Holder by his
acceptance hereof agrees to be bound by such provisions and authorizes and
expressly directs the Trustee, on his behalf, to take such action as may be
necessary or appropriate to effectuate the subordination provided for in the
Indenture and appoints the Trustee his attorney-in-fact for such purposes.

          6.   Redemption.
               ----------

          (a) Optional Redemption. The Company may redeem the Notes, at its
              -------------------
option, in whole at any time or in part from time to time, on and after April 1,
2005 at the following Redemption Prices (expressed as percentages of the
principal amount thereof) if redeemed during the twelve-month period commencing
on April 1 of the year set forth below, plus, in each case, accrued and unpaid
interest thereon, if any, to the date of redemption.

     Year                                        Percentage
     ----                                        ----------
     2005...................................     105.125%
     2006...................................     103.417%
     2007...................................     101.708%
     2008 and thereafter....................     100.000%

          (b) Optional Redemption Upon Equity Offerings.  Notwithstanding the
              -----------------------------------------
foregoing, the Company may redeem in the aggregate up to 35% of the original
principal amount of Notes at any time and from time to time prior to April 1,
2003 at a Redemption Price equal to 110.25% of the aggregate principal amount so
redeemed, plus accrued and unpaid interest, if any, to the Redemption Date out
of the Net Proceeds of one or more Public Equity Offerings; provided that

          (1) at least 65% of the principal amount of Notes originally issued
remains outstanding immediately after the occurrence of any such redemption and

          (2) any such redemption occurs within 90 days following the closing of
any such Public Equity Offering.

                                      B-4
<PAGE>

          7.   Notice of Redemption. Notice of redemption under paragraphs 6(a)
               --------------------
and (b) of this Note will be mailed at least 30 days but not more than 60 days
before the Redemption Date to each Holder to be redeemed at such Holder's
registered address.

          Except as set forth in the Indenture, if monies for the redemption of
the Notes called for redemption shall have been deposited with the Paying Agent
for redemption on such Redemption Date, then, unless the Company defaults in the
payment of such Redemption Price plus accrued interest, if any, the Notes called
for redemption will cease to bear interest from and after such Redemption Date
and the only right of the Holders of such Notes will be to receive payment of
the Redemption Price plus accrued interest, if any.

          8.   Offers to Purchase.  The Indenture provides that, after certain
               ------------------
Asset Sales (as defined in the Indenture) and upon the occurrence of a Change of
Control (as defined in the Indenture), and subject to further limitations
contained therein, the Company will make an offer to purchase certain amounts of
the Notes in accordance with the procedures set forth in the Indenture.

          9.   Denominations; Transfer; Exchange.  The Notes are in registered
               ---------------------------------
form, without coupons, in denominations of $1,000 and integral multiples
thereof.  A Holder shall register the transfer or exchange of Notes in
accordance with the Indenture.  The Registrar may require a Holder, among other
things, to furnish appropriate endorsements and transfer documents and to pay
certain transfer taxes or similar governmental charges payable in connection
therewith as permitted by the Indenture.  The Registrar need not register the
transfer of or exchange of any Notes or portions thereof selected for
redemption.

          10.  Persons Deemed Owners.  The registered holder of a Note shall be
               ---------------------
treated as the owner of it for all purposes.

          11.  Unclaimed Money.  If money for the payment of principal or
               ---------------
interest remains unclaimed for two years, the Trustee and the Paying Agent will
pay the money back to the Company upon written request.  After that, Holders
entitled to money must look to the Company for payment as general creditors
unless an "abandoned property" law designates another person.

          12.  Legal Defeasance and Covenant Defeasance.  If the Company at any
               ----------------------------------------
time deposits with the Trustee U.S. legal tender or U.S. Government Obligations
sufficient to pay the principal of and interest on the Notes to redemption or
maturity and complies with the other provisions of the Indenture relating to
defeasance, the Company will be discharged from certain provisions of the
Indenture and the Notes (including certain covenants, but excluding its
obligation to pay the principal of and interest on the Notes).

          13.  Amendments, Supplements, and Waivers.  Subject to certain
               ------------------------------------
exceptions, the Indenture or the Notes may be amended or supplemented with the
written consent of

                                      B-5
<PAGE>

the Holders of at least a majority in aggregate principal amount of the Notes
then outstanding, and any existing Default or Event of Default or noncompliance
with any provision may be waived with the written consent of the Holders of a
majority in aggregate principal amount of the Notes then outstanding. Without
notice to or consent of any Holder, the parties thereto may amend or supplement
the Indenture or the Notes to, among other things, cure any ambiguity, defect or
inconsistency, provide for uncertificated Notes in addition to or in place of
certificated Notes or make any other change that does not adversely affect in
any material respect the rights of any Holder.

          14.  Restrictive Covenants.  The Indenture imposes certain limitations
               ---------------------
on the ability of the Company and its Subsidiaries to, among other things, incur
additional Indebtedness, make payments in respect of its Capital Stock, enter
into transactions with Affiliates, create dividend or other payment restrictions
affecting Restricted Subsidiaries, sell assets, create liens, issue capital
stock, enter into sale and lease-back transactions, make certain Investments,
merge or consolidate with any other Person, or sell, assign, transfer, lease,
convey or otherwise dispose of all or substantially all of its assets.  Such
limitations are subject to a number of important qualifications and exceptions.
The Company must annually report to the Trustee on compliance with such
limitations.

          15.  Successor Entity.  When a successor entity assumes, in accordance
               ----------------
with the Indenture, all the obligations of its predecessor under the Notes and
the Indenture, and immediately before and thereafter no Default or Event of
Default exists and certain other conditions are satisfied, the predecessor
entity will be released from those obligations.

          16.  Defaults and Remedies.  Events of Default are set forth in the
               ---------------------
Indenture.  If an Event of Default (other than an Event of Default pursuant to
Section 6.01(f) or (g) of the Indenture) shall have occurred and be continuing,
then the Trustee or the Holders of not less than 25% in aggregate principal
amount of the Notes then outstanding, may declare to be immediately due and
payable the entire principal amount of all the Notes then outstanding plus
accrued interest to the date of acceleration; provided, however, that after such
acceleration but before a judgment or decree based on such acceleration is
obtained by the Trustee, the Holders of a majority in aggregate principal amount
of the outstanding Notes may rescind and annul such acceleration and its
consequences if all existing Events of Default, other than the nonpayment of
principal, premium, if any, or interest that has become due solely because of
the acceleration, have been cured or waived. No such rescission shall affect any
subsequent Default or impair any right consequent thereto.  In case an Event of
Default specified in Section 6.01(f) or (g) of the Indenture occurs, such
principal amount, together with premium, if any, and interest with respect to
all of the Notes, shall be due and payable immediately without any declaration
or other act on the part of the Trustee or the Holders.

          17.  Trustee Dealings with Company.  The Trustee under the Indenture,
               -----------------------------
in its individual or any other capacity, may make loans to, accept deposits
from, and perform

                                      B-6
<PAGE>

services for the Company, and may otherwise deal with the Company, its
Subsidiaries or their respective Affiliates as if it were not the Trustee.

          18.  No Recourse Against Others.  As more fully described in the
               --------------------------
Indenture, no director, officer, employee, stockholder or incorporator, as such,
of the Company shall have any liability for any obligation of the Company under
the Notes or the Indenture or for any claim based on, in respect of or by reason
of such obligations or their creation.  Each Holder by accepting a Note waives
and releases all such liability.  Such waiver and release are part of the
consideration for the issuance of the Notes.

          19.  Authentication.  This Note shall not be valid until the Trustee
               --------------
or Authenticating Agent manually signs the certificate of authentication on this
Note.

          20.  Governing Law.  THIS NOTE SHALL BE GOVERNED BY AND CONSTRUED IN
               -------------
ACCORDANCE WITH THE LAWS OF THE STATE OF NEW YORK, AS APPLIED TO CONTRACTS MADE
AND PERFORMED WITHIN THE STATE OF NEW YORK, WITHOUT REGARD TO PRINCIPLES OF
CONFLICTS OF LAW.  EACH OF THE PARTIES TO THE INDENTURE HAS AGREED TO SUBMIT TO
THE JURISDICTION OF THE COURTS OF THE STATE OF NEW YORK IN ANY ACTION OR
PROCEEDING ARISING OUT OF OR RELATING TO THIS NOTE.

          21.  Abbreviations and Defined Terms.  Customary abbreviations may be
               -------------------------------
used in the name of a Holder or an assignee, such as:  TEN COM (= tenants in
common), TEN ENT (= tenants by the entireties), JT TEN (= joint tenants with
right of survivorship and not as tenants in common), CUST (= Custodian), and
U/G/M/A (= Uniform Gifts to Minors Act).

          22.  CUSIP Numbers.  Pursuant to a recommendation promulgated by the
               -------------
Committee on Uniform Security Identification Procedures, the Company has caused
CUSIP numbers to be printed on the Notes as a convenience to the Holders.  No
representation is made as to the accuracy of such numbers as printed on the
Notes and reliance may be placed only on the other identification numbers
printed hereon.

          23.  Indenture.  Each Holder, by accepting a Note, agrees to be bound
               ---------
by all of the terms and provisions of the Indenture, as the same may be amended
from time to time.

          The Company will furnish to any Holder upon written request and
without charge a copy of the Indenture, which has the text of this Note in
larger type.  Requests may be made to:  TNP Enterprises, Inc., c/o Laurel Hill
Capital Partners LLC, 2 Robbins Lane, Suite 201, Jericho, New York 11753,
Attention:  Kathleen Marion.

                                      B-7
<PAGE>

                                ASSIGNMENT FORM

          If you the Holder want to assign this Note, fill in the form below and
have your signature guaranteed:

I or we assign and transfer this Note to:

________________________________________________________________________________

________________________________________________________________________________

________________________________________________________________________________
                  (Print or type name, address and zip code and
                  social security or tax ID number of assignee)

and irrevocably appoint _______________________________________________________,
agent to transfer this Note on the books of the Company.  The agent may
substitute another to act for him.

Date: __________________    Signed: ___________________________________________
                                     (Sign exactly as your name appears on the
                                     other side of this Note)

Medallion Guarantee: _____________________

                                      B-8
<PAGE>

                     [OPTION OF HOLDER TO ELECT PURCHASE]

          If you want to elect to have this Note purchased by the Company
pursuant to Section 4.13 or Section 4.16 of the Indenture, check the appropriate
box:

                                Section 4.13 [_]

                                Section 4.16 [_]

          If you want to elect to have only part of this Note purchased by the
Company pursuant to Section 4.13 or Section 4.16 of the Indenture, state the
amount you elect to have purchased:

$ __________________

Date: __________________
                             _______________________________________________
                             NOTICE:  The signature on this assignment must
                             correspond with the name as it appears upon the
                             face of the within Note in every particular without
                             alteration or enlargement or any change whatsoever
                             and be guaranteed by the endorser's bank or broker.

Medallion Guarantee: _____________________

                                      B-9
<PAGE>

                                                                       EXHIBIT C
                                                                       ---------
                   CERTIFICATE TO BE DELIVERED UPON EXCHANGE
                      OR REGISTRATION OF TRANSFER OF NOTES

Re:  TNP ENTERPRISES, INC. (the "Company")
                                 -------
          10.25% Senior Subordinated Notes due 2010 (the "Notes")
          -------------------------------------------------------

          This Certificate relates to $_______ principal amount of Notes held in
the form of* ___ a beneficial interest in a Global Note or* _______ Certificated
Notes by ______ (the "Transferor").
                      ----------

The Transferor:

          [_]  has requested by written order that the Registrar deliver in
exchange for its beneficial interest in the Global Note held by the Depository a
Certificated Note or Certificated Notes in definitive, registered form of
authorized denominations and an aggregate number equal to its beneficial
interest in such Global Note (or the portion thereof indicated above); or

          [_]  has requested by written order that the Registrar exchange or
register the transfer of a Certificated Note or Certificated Notes.

          In connection with such request and in respect of each such Note, the
Transferor does hereby certify that the Transferor is familiar with the
Indenture relating to the above captioned Notes and the restrictions on
transfers thereof as provided in Section 2.16 of such Indenture, and that the
transfer of the Notes does not require registration under the Securities Act of
1933, as amended (the "Securities Act"), because*:
                       --------------

          [_]  Such Note is being acquired for the Transferor's own account,
without transfer (in satisfaction of Section 2.16 of the Indenture).

          [_]  Such Note is being transferred to a "qualified institutional
buyer" (as defined in Rule 144A under the Securities Act), in reliance on Rule
144A.

          [_]  Such Note is being transferred to an institutional "accredited
investor" (within the meaning of subparagraph (a)(1), (2), (3) or (7) of Rule
501 under the Securities Act) which delivers a certificate to the Trustee in the
form of Exhibit D to the Indenture.
        ---------

          [_]  Such Note is being transferred in reliance on Regulation S under
the Securities Act and a transfer certificate for Regulation S transfers in the
form of Exhibit E to the Indenture accompanies this certification.  [An Opinion
        ---------
of Counsel to the effect that such transfer does not require registration under
the Securities Act accompanies this certification.]

          [_]  Such Note is being transferred in reliance on Rule 144 under the
Securities Act.  [An Opinion of Counsel to the effect that such transfer does
not require registration under the Securities Act accompanies this
certification.]

                                 C-1
<PAGE>

          [_]  Such Note is being transferred in reliance on and in compliance
with an exemption from the registration requirements of the Securities Act other
than Rule 144A or Rule 144 under the Securities Act to a person other than an
institutional "accredited investor."  [An Opinion of Counsel to the effect that
such transfer does not require registration under the Securities Act accompanies
this certification.]

                                           _________________________________
                                            [INSERT NAME OF TRANSFEROR]

                                            By: ____________________________
                                                   [Authorized Signatory]

Date: _________________
*Check applicable box.

                                      C-2
<PAGE>

                                                                       EXHIBIT D
                                                                       ---------

                  Form of Transferee Letter of Representation

THE BANK OF NEW YORK
101 Barclay Street, Floor 21 West
New York, New York  10286

Attention:  Corporate Trust Trustee Administration

Ladies and Gentlemen:

          This certificate is delivered to request a transfer of $________
principal amount of the 10.25% Senior Subordinated Notes due 2010 of TNP
ENTERPRISES, INC., (the "Company") and any guarantee thereof (the "Notes").
                         -------                                   -----
Upon transfer, the Notes would be registered in the name of the new beneficial
owner as follows:

          Name:______________________________
          Address:___________________________
          Taxpayer ID Number:________________

          The undersigned represents and warrants to you that:

          1.   We are an institutional "accredited investor" (as defined in Rule
501(a)(1), (2), (3) or (7) under the Securities Act of 1933 (the "Securities
                                                                  ----------
Act")) purchasing Notes for our own account or for the account of such an
---
institutional "accredited investor" and we are acquiring the Notes not with a
view to, or for offer or sale in connection with, any distribution in violation
of the Securities Act.  We have such knowledge and experience in financial and
business matters as to be capable of evaluating the merits and risk of our
investment in the Notes and we invest in or purchase securities similar to the
Notes in the normal course of our business.  We and any accounts for which we
are acting are each able to bear the economic risk of our or its investment.

          2.   We acknowledge that we have had access to such financial and
other information, and have been afforded the opportunity to ask such questions
of representatives of the Company and receive answers thereto, as we deem
necessary.

          3.   We understand that the Notes have not been registered under the
Securities Act and, unless so registered, may not be sold except as permitted in
the following sentence.  We agree on our own behalf and on behalf of any
investor account for which we are purchasing Notes that we will not prior to the
date (the "Resale Restriction Termination Date") that is two years after the
           -----------------------------------
later of the original issuance of the Notes and the last date on which the
Company or any affiliate of the Company was the owner of such Notes (or any
predecessor thereto) offer, sell or otherwise transfer such Notes except (a) to
the Company or any subsidiary of the Company, (b) inside the United States to a
"qualified institutional buyer" in compliance with Rule 144A under the
Securities Act (c) inside the United States to an "institutional accredited
investor" as defined in Rule 501(a)(1), (2), (3) or (7) under the Securities

                                      D-1
<PAGE>

Act that, prior to such transfer, furnishes (or has furnished on its behalf by a
U.S. broker-dealer) to the Trustee a signed letter substantially in the form of
this letter (d) outside the United States in an offshore transaction in
compliance with Rule 904 under the Securities Act (e) pursuant to any other
available exemption from the registration requirements of the Securities Act or
(f) pursuant to an effective registration statement under the Securities Act.
We acknowledge that the Company and the Trustee reserve the right prior to any
offer, sale or other transfer prior to the Resale Restriction Termination Date
of the applicable Notes pursuant to clause (c) or (e) above to require the
delivery of an opinion of counsel, certification and/or other information
satisfactory to the Company and the Trustee.

          We understand that the Trustee will not be required to accept for
registration of transfer any Notes acquired by us, except upon presentation of
evidence satisfactory to the Company and the Trustee that the foregoing
restrictions on transfer have been complied with.  We further understand that
any Notes purchased by us will be in the form of definitive physical
certificates and that such certificates will bear a legend reflecting the
substance of  paragraph 3 of this letter.  We further agree to provide to any
person acquiring any of the Notes from us a notice advising such person that
transfers of such Notes are restricted as stated herein and that certificates
representing such Notes will bear a legend to that effect.

          We represent that the Company and the Trustee and others are entitled
to rely upon the truth and accuracy of our acknowledgments, representations and
agreements set forth herein, and we agree to notify you promptly in writing if
any of our acknowledgments, representations or agreements herein cease to be
accurate and complete.  You are also irrevocably authorized to produce this
letter or a copy hereof to any interested party in any administrative or legal
proceeding or official inquiry with respect to the matters covered hereby.

          We represent to you that we have full power to make the foregoing
acknowledgments, representations and agreements on our own behalf and on behalf
of any investor account for which we are acting as fiduciary agent.

          As used herein, the terms "offshore transaction," "United States" and
"U.S. person" have the respective meanings given to them in Regulation S under
the Securities Act.

                                      D-2
<PAGE>

          THIS LETTER SHALL BE GOVERNED BY, AND CONSTRUED IN ACCORDANCE WITH,
THE LAWS OF THE STATE OF NEW YORK.



Dated:  __________             TRANSFEREE:

                                         By:_____________________


                                      D-3
<PAGE>

                                                                       EXHIBIT E

                                                                       ---------
                           Form of Certificate To Be
                            Delivered in Connection
                          with Regulation S Transfers

                                                           _______________, ____

THE BANK OF NEW YORK
101 Barclay Street, Floor 21 West
New York, New York 10286

Attention:  Corporate Trust Trustee Administration

Re:  TNP Enterprises, Inc. 10.25% Senior
     Subordinated Notes due 2010 (the "Notes")
     -----------------------------------------

Ladies and Gentlemen:

          In connection with our proposed sale of $__________ aggregate
principal amount of the Notes, we confirm that such sale has been effected
pursuant to and in accordance with Regulation S under the Securities Act of
1933, as amended (the "Securities Act"), and, accordingly, we represent that:
                       --------------

          (1) the offer of the Notes was not made to a person in the United
     States;

          (2) either (a) at the time the buy offer was originated, the
     transferee was outside the United States or we and any person acting on our
     behalf reasonably believed that the transferee was outside the United
     States, or (b) the transaction was executed in, on or through the
     facilities of a designated off-shore securities market and neither we nor
     any person acting on our behalf knows that the transaction has been
     prearranged with a buyer in the United States;

          (3) no directed selling efforts have been made in the United States in
     contravention of the requirements of Rule 903(a) or Rule 904(a) of
     Regulation S, as applicable;

          (4) the transaction is not part of a plan or scheme to evade the
     registration requirements of the Securities Act; and

          (5) we have advised the transferee of the transfer restrictions
     applicable to the Notes.

                                      E-1
<PAGE>

          You and the Company are entitled to rely upon this letter and are
irrevocably authorized to produce this letter or a copy hereof to any interested
party in any administrative or legal proceedings or official inquiry with
respect to the matters covered hereby.  Defined terms used herein without
definition have the respective meanings provided in Regulation S.

                                          Very truly yours,


                                          [Name of Transferor]


                                          By:_______________________

                                      E-2
<PAGE>

                                                                       EXHIBIT F
                                                                       ---------

                        FORM OF SUPPLEMENTAL INDENTURE
                        ------------------------------

          SUPPLEMENTAL INDENTURE (this "Supplemental Indenture"), dated as of,
among (the "New Guarantor"), a subsidiary of TNP Enterprises, Inc. (or its
            -------------
successor), a Texas corporation (the "Company"), the Guarantors, if any (the
                                      -------
"Existing Guarantors"), under the Indenture referred to below, and The Bank
 -------------------
of New York, as trustee under the Indenture referred to below (the "Trustee").
                                                                    -------

                             W I T N E S S E T H :
                             - - - - - - - - - -

          WHEREAS the Company has heretofore executed and delivered to the
Trustee an Indenture (as such may be amended from time to time, the
"Indenture"), dated as of April 7, 2000, providing for the issuance of its
10.25% Senior Subordinated Notes due 2010 (the "Notes");

          WHEREAS Section 4.22 of the Indenture provides that under certain
circumstances the Company is required to cause the New Guarantor to execute and
deliver to the Trustee a supplemental indenture pursuant to which the New
Guarantor shall unconditionally guarantee all of the Company's obligations under
the Notes pursuant to a Guarantee on the terms and conditions set forth herein;
and

          WHEREAS pursuant to Section 8.01 of the Indenture, the Trustee, the
Company and Existing Guarantors are authorized to execute and deliver this
Supplemental Indenture;

          NOW THEREFORE, in consideration of the foregoing and for other good
and valuable consideration, the receipt of which is hereby acknowledged, the New
Guarantor, the Company, the Existing Guarantors and the Trustee mutually
covenant and agree for the equal and ratable benefit of the Holders as follows:

          1.   Definitions.  (a) Capitalized terms used herein without
               -----------
definition shall have the meanings assigned to them in the Indenture.

          (b)  For all purposes of this Supplemental Indenture, except as
otherwise herein expressly provided or unless the context otherwise requires:
(i) the terms and expressions used herein shall have the same meanings as
corresponding terms and expressions used in the Indenture; and (ii) the words
"herein," "hereof" and "hereby" and other words of similar import used in this
Supplemental Indenture refer to this Supplemental Indenture as a whole and not
to any particular section hereof.

                                      C-3
<PAGE>

          2.   Agreement to Guarantee.  The New Guarantor hereby agrees, jointly
               ----------------------
and severally with all other Guarantors, to Guarantee the Company's obligations
under the Notes on the terms and subject to the conditions set forth in Article
10 of the Indenture and to be bound by all other applicable provisions of the
Indenture.  From and after the date hereof, the New Guarantor shall be a
Guarantor for all purposes under the Indenture and the Notes.

          3.   Ratification of Indenture; Supplemental Indentures Part of
               ----------------------------------------------------------
Indenture.  Except as expressly amended hereby, the Indenture is in all respects
---------
ratified and confirmed and all the terms, conditions and provisions thereof
shall remain in full force and effect.  This Supplemental Indenture shall form a
part of the Indenture for all purposes, and every Holder heretofore or hereafter
authenticated and delivered shall be bound hereby.

          4.   Governing Law.  THIS SUPPLEMENTAL INDENTURE SHALL BE GOVERNED BY
               -------------
AND CONSTRUED IN ACCORDANCE WITH THE LAWS OF THE STATE OF NEW YORK, AS APPLIED
TO CONTRACTS MADE AND PERFORMED WITHIN THE STATE OF NEW YORK, WITHOUT REGARD TO
PRINCIPLES OF CONFLICTS OF LAW.  EACH OF THE PARTIES HERETO AGREES TO SUBMIT TO
THE JURISDICTION OF THE COURTS OF THE STATE OF NEW YORK IN ANY ACTION OR
PROCEEDING ARISING OUT OF OR RELATING TO THIS SUPPLEMENTAL INDENTURE.

          5.   Trustee Makes No Representation.  The Trustee shall not be
               -------------------------------
responsible in any manner whatsoever for or in respect of the validity or
sufficiency of this Supplemental Indenture or for or in respect of the recitals
contained herein, all of which are made solely by the Company.

          6.   Multiple Counterparts.  The parties may sign multiple
               ---------------------
counterparts of this Supplemental Indenture.  Each signed counterpart shall be
deemed an original, but all of them together represent one and the same
agreement.

          7.   Headings.  The headings of this Supplemental Indenture have been
               --------
inserted for convenience of reference only, are not to be considered a part
hereof, and shall in no way modify or restrict any of the terms or provisions
hereof.


                                      F-4
<PAGE>

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

                                             [NEW GUARANTOR]

                                             By:______________________
                                                Name:
                                                Title:

                                             By:______________________
                                                Name:
                                                Title:

                                             TNP ENTERPRISES, INC.

                                             By:______________________
                                                Name:
                                                Title:

                                             By:______________________
                                                Name:
                                                Title:


                                      F-5
<PAGE>

                                             EXISTING GUARANTORS:

                                             By:______________________
                                                Name:
                                                Title:

                                             By:______________________
                                                Name:
                                                Title:

                                             THE BANK OF NEW YORK,
                                             as Trustee

                                             By:______________________
                                                Name:
                                                Title:

                                      F-6