Document:

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                                                                    EXHIBIT 4.20

                          FIFTH SUPPLEMENTAL INDENTURE

         FIFTH SUPPLEMENTAL INDENTURE dated as of March 27, 2000 (this
"SUPPLEMENTAL SENIOR SUBORDINATED NOTE INDENTURE") among P&L Coal Holdings
Corporation, a Delaware corporation (the "Company"), each Senior Subordinated
Note Guarantor (as defined in the Senior Subordinated Note Indenture referred to
herein) and State Street Bank and Trust Company, as Senior Subordinated Note
Trustee under the Senior Subordinated Note Indenture referred to below (the"
SENIOR NOTE TRUSTEE").

                                   WITNESSETH

         WHEREAS, the Company has heretofore executed and delivered to the
Senior Subordinated Note Trustee a Senior Subordinated Note Indenture dated as
of May 18, 1998 (the "SENIOR SUBORDINATED NOTE INDENTURE") providing for the
issuance of an aggregate principal amount of up to $650.0 million of 9-5/8%
Senior Subordinated Notes due 2008 (the "Senior Subordinated Notes");

         WHEREAS, the Company desires to clarify the definition of "Lien" in
Section 1.01 of the Senior Subordinated Note Indenture;

         WHEREAS, Section 9.01 of the Senior Subordinated Note Indenture
provides that the Company, the Senior Subordinated Note Guarantors and the
Senior Subordinated Note Trustee may enter into a supplemental indenture,
without the consent of any Holders, for the purpose of curing any ambiguity,
defect or inconsistency or for the purpose of making any change that does not
adversely affect the Holders of the Senior Subordinated Notes;

         NOW, THEREFORE, in consideration of the foregoing and for other good
and valuable consideration, the receipt of which is hereby acknowledged, the
Company, the Senior subordinated Note Guarantors and the Senior Subordinated
Note Trustee hereby agree as follows:

         1.       Capitalized Terms. Capitalized terms used herein without
                  definition shall have the meanings assigned to them in the
                  Senior Subordinated Note Indenture.

         2.       Amendment to Senior Subordinated Note Indenture. The
                  definition of "Lien" in Section 1.01 of the Senior Subordinate
                  Note Indenture is hereby amended by inserting the following
                  clause at the end thereof: ",but excluding any of the
                  foregoing arising as a result of a sale, contribution,
                  disposition or other transfer of accounts, chattel paper,
                  payment intangibles, promissory notes and/or related assets
                  otherwise permitted under the terms hereof'.

         3.       NEW YORK LAW TO GOVERN. THE INTERNAL LAW OF THE STATE OF NEW
                  YORK SHALL GOVERN AND BE USED TO CONSTUE THIS SUPPLEMENTAL
                  SENIOR SUBORDINATED NOTE INDENTURE BUT WITHOUT GIVING EFFECT
                  TO APPLICABLE PRINCIPLES OF CONFLICTS OF LAW TO THE EXTENT
                  THAT THE APPLICATION OF THE LAWS OF ANOTHER JURISDICTION WOULD
                  BE REQUIRED THEREBY.

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         4.       Counterparts. The parties may sign any number of copies of
this supplemental Senior subordinated Note Indenture. Each signed copy shall be
an original, but all of them together represent the same agreement.

         5.       Effect of Headings. The Section headings herein are for
convenience only and shall not affect the construction hereof.

         6.       The Senior Subordinated Note Trustee. The Senior Subordinated
Note Trustee shall not be responsible in any manner whatsoever for or in respect
of the validity or sufficiency of this Supplemental Senior subordinated Note
Indenture or for or in respect of the recitals contained herein, all of which
recitals are made solely by the Company and the Senior Subordinated Note
Guarantors.

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

                                 P&L COAL HOLDING CORPORATION

                                 By:    /s/  J. L. Klinger
                                      -----------------------------------------
                                 Name:  J. L. Klinger
                                 Title:  Vice President-Legal Services
                                 And Secretary

                                 STATE STREET BANK AND TRUST COMPANY,
                                 As Trustee

                                 By:  /s/ Philip G. Kane, Jr.
                                      -----------------------------------------
                                 Name:  Philip G. Kane, Jr.
                                 Title:  Vice President

                                 AFFINITY MINING COMPANY

                                 By:   /s/ J. C. Klingl
                                      -----------------------------------------
                                 Name:  J. C. Klingl
                                 Title:  Vice President

                                 ARID OPERATIONS INC.

                                 By:    /s/ Steven F. Schaab
                                      -----------------------------------------
                                 Name:  S. F. Schaab
                                 Title:  Vice President and Treasurer

                                 BIG SKY COAL COMPANY

                                 By:     /s/ J. C. Klingl
                                      -----------------------------------------
                                 Name:  J. C. Klingl
                                 Title:  Vice President

                                 BLACKROCK FIRST CAPITAL CORPORATION

                                 By:   /s/  J. C. Sevem
                                      -----------------------------------------
                                 Name:  J. C. Sevem
                                 Title:  Vice President

                                 BLUEGRASS COAL COMPANY

                                 By:     /s/  Steven F. Schaab
                                      -----------------------------------------
                                 Name:  S. F. Schaab
                                 Title:  Vice President and Treasurer

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                                 CABALLO COAL COMPANY

                                 By:     /s/  Steven F. Schaab
                                      -----------------------------------------
                                 Name:  S. F. Schaab
                                 Title:  Vice President and Treasurer

                                 CHARLES COAL COMPANY

                                 By:  /s/  J. C. Sevem
                                      -----------------------------------------
                                 Name:  J. C. Sevem
                                 Title:  Vice President

                                 COAL PROPERTIES CORP.

                                 By:     /s/  Steven F. Schaab
                                      -----------------------------------------
                                 Name:  S. F. Schaab
                                 Title:  Vice President and Treasurer

                                 COLONY BAY COAL COMPANY
                                 By:  Charles Coal Company

                                 By:   /s/  J. C. Sevem
                                      -----------------------------------------
                                 Name:  J. C. Sevem
                                 Title:  Vice President

                                 COOK MOUNTAIN COAL COMPANY

                                 By:     /s/  Steven F. Schaab
                                      -----------------------------------------
                                 Name:  S. F. Schaab
                                 Title:  Vice President and Treasurer

                                 COTTONWOOD LAND COMPANY

                                 By:     /s/  Steven F. Schaab
                                      -----------------------------------------
                                 Name:  S. F. Schaab
                                 Title:  Vice President and Treasurer

                                 DARIUS GOLD MINE, INC.

                                 By:     /s/  Steven F. Schaab
                                      -----------------------------------------
                                 Name:  S. F. Schaab
                                 Title:  Vice President and Treasurer

                                 EACC CAMPS, INC.

                                 By:  /s/ J. C. Sevem
                                      -----------------------------------------
                                 Name:  J. C. Sevem
                                 Title:  Vice President

                                       4
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                                 EASTERN ASSOCIATED COAL CORP.

                                 By:     /s/  Steven F. Schaab
                                      -----------------------------------------
                                 Name:  S. F. Schaab
                                 Title:  Vice President

                                 EASTERN ROYALTY CORP.

                                 By:  /s/  J. C. Sevem
                                      -----------------------------------------
                                 Name:  J. C. Sevem
                                 Title:  Vice President

                                 GOLDFIELDS CHILE, S.A.

                                 By:     /s/  Steven F. Schaab
                                      -----------------------------------------
                                 Name:  S. F. Schaab
                                 Title:  Vice President

                                 GOLDFIELDS MINING  CORPORATION

                                 By:     /s/  Steven F. Schaab
                                      -----------------------------------------
                                 Name:  S. F. Schaab
                                 Title:  Vice President and Treasurer

                                 GOLDFIELDS OPERATING CO. - ORTIZ

                                 By:     /s/  Steven F. Schaab
                                      -----------------------------------------
                                 Name:  S. F. Schaab
                                 Title:  Vice President and Treasurer

                                 GRAND EAGLE MINING, INC.

                                 By:     /s/  Steven F. Schaab
                                      -----------------------------------------
                                 Name:  S. F. Schaab
                                 Title:  Vice President

                                 HAYDEN GULCH TERMINAL, INC.

                                 By:     /s/  Steven F. Schaab
                                      -----------------------------------------
                                 Name:  S. F. Schaab
                                 Title:  Vice President and Treasurer

                                 HIGHLAND MINING COMPANY

                                 By:  /s/  J. C. Klingl
                                      -----------------------------------------
                                 Name:  J. C. Klingl
                                 Title:  Vice President

                                 INDEPENDENCE MATERIAL HANDLING
                                 COMPANY

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                                 By:     /s/  Steven F. Schaab
                                      -----------------------------------------
                                 Name:  S. F. Schaab
                                 Title:  Vice President and Treasurer

                                 INTERIOR HOLDING CORP.

                                 By:     /s/  Steven F. Schaab
                                      -----------------------------------------
                                 Name:  S. F. Schaab
                                 Title:  Vice President and Treasurer

                                 JAMES RIVER COAL TERMINAL COMPANY

                                 By:     /s/  Steven F. Schaab
                                      -----------------------------------------
                                 Name:  S. F. Schaab
                                 Title:  Vice President and Treasurer

                                 JUNIPER COAL COMPANY

                                 By:     /s/  Steven F. Schaab
                                      -----------------------------------------
                                 Name:  S. F. Schaab
                                 Title:  Vice President and Treasurer

                                 KAYENTA MOBILE HOME PARK, INC.

                                 By:  /s/  D. A. Wagner
                                      -----------------------------------------
                                 Name:  Doug Wagner
                                 Title:  President

                                 MARTINKA COAL COMPANY

                                 By:  /s/  J. C. Klingl
                                      -----------------------------------------
                                 Name:  J. C. Klingl
                                 Title:  Vice President

                                 MIDCO SUPPLY AND EQUIPMENT
                                 CORPORATION

                                 By:     /s/  Steven F. Schaab
                                      -----------------------------------------
                                 Name:  S. F. Schaab
                                 Title:  Vice President and Treasurer

                                 MOUNTAIN VIEW COAL COMPANY

                                 By:  /s/  J. C. Klingl
                                      -----------------------------------------
                                 Name:  J. C. Klingl
                                 Title:  Vice President

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                                 NORTH PAGE COAL CORP.

                                 By:  /s/  J. C. Sevem
                                      -----------------------------------------
                                 Name:  J. C. Sevem
                                 Title:  Vice President

                                 OHIO COUNTY COAL COMPANY

                                 By:     /s/  Steven F. Schaab
                                      -----------------------------------------
                                 Name:  S. F. Schaab
                                 Title:  Vice President and Treasurer

                                 PATRIOT COAL COMPANY L.P.
                                 By:  Bluegrass Coal Company

                                 By:     /s/  Steven F. Schaab
                                      -----------------------------------------
                                 Name:  S. F. Schaab
                                 Title:  Vice President and Treasurer

                                 PEABODY AMERICA, INC.

                                 By:  /s/  J. L. Klinger
                                      -----------------------------------------
                                 Name:  J. L. Klinger
                                 Title:  Vice President and Assistant Secretary

                                 PEABODY COALSALES COMPANY

                                 By: /s/ J. C. Sevem
                                     ------------------------------------------
                                 Name:  J. C. Sevem
                                 Title:  Secretary

                                 PEABODY COALTRADE, INC.

                                 By:     /s/  Steven F. Schaab
                                      -----------------------------------------
                                 Name:  S. F. Schaab
                                 Title:  Vice President and Treasurer

                                 PEABODY COAL COMPANY

                                 By:  /s/  J. C. Klingl
                                      -----------------------------------------
                                 Name:  J. C. Klingl
                                 Title:  Vice President
                                                                     `
                                 PEABODY DEVELOPMENT COMPANY

                                 By:     /s/  Steven F. Schaab
                                      -----------------------------------------
                                 Name:  S. F. Schaab
                                 Title:  Vice President and Treasurer

                                 PEABODY DEVELOPMENT LAND HOLDING, LLC

                                 By:     /s/  Steven F. Schaab
                                      -----------------------------------------
                                 Name:  S. F. Schaab
                                 Title:  Vice President and Treasurer

                                 PEABODY ENERGY SOLUTIONS, INC.

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                                 By:     /s/  Steven F. Schaab
                                      -----------------------------------------
                                 Name:  S. F. Schaab
                                 Title:  Vice President and Treasurer

                                 PEABODY ENTERPRISES, INC. I

                                 By:     /s/  Steven F. Schaab
                                      -----------------------------------------
                                 Name:  S. F. Schaab
                                 Title:  Vice President

                                 PEABODY ENTERPRISES, INC. II

                                 By:     /s/  Steven F. Schaab
                                      -----------------------------------------
                                 Name:  S. F. Schaab
                                 Title:  Vice President

                                 PEABODY HOLDING COMPANY, INC.

                                 By:     /s/  Steven F. Schaab
                                      -----------------------------------------
                                 Name:  S. F. Schaab
                                 Title:  Vice President and Treasurer

                                 PEABODY NATURAL GAS, LLC

                                 By:     /s/  Steven F. Schaab
                                      -----------------------------------------
                                 Name:  S. F. Schaab
                                 Title:  Vice President and Treasurer

                                 PEABODY NATURAL RESOURCES COMPANY
                                 By:  Gold Fields Mining Corporation

                                 By:     /s/  Steven F. Schaab
                                      -----------------------------------------
                                 Name:  S. F. Schaab
                                 Title:  Vice President and Treasurer

                                 PEABODY SOUTHWESTERN COAL COMPANY

                                 By:     /s/  Steven F. Schaab
                                      -----------------------------------------
                                 Name:  S. F. Schaab
                                 Title:  Vice President and Assistant Treasurer

                                 PEABODY TERMINALS, INC.

                                 By:     /s/  Steven F. Schaab
                                      -----------------------------------------
                                 Name:  S. F. Schaab
                                 Title:  Vice President and Treasurer

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<PAGE>
                                 PEABODY VENEZUELA COAL CORP.

                                 By:     /s/  Steven F. Schaab
                                      -----------------------------------------
                                 Name:  S. F. Schaab
                                 Title:  Vice President and Treasurer

                                 PEABODY WESTERN COAL COMPANY

                                 By:  /s/ J. C. Klingl
                                      -----------------------------------------
                                 Name:  J. C. Klingl
                                 Title:  Vice President

                                 PINE RIDGE COAL COMPANY

                                 By:  /s/ J. C. Klingl
                                      -----------------------------------------
                                 Name:  J. C. Klingl
                                 Title:  Vice President

                                 PORCUPINE PRODUCTION, LLC

                                 By:     /s/  Steven F. Schaab
                                      -----------------------------------------
                                 Name:  S. F. Schaab
                                 Title:  Vice President and Treasurer

                                 PORCUPINE TRANSPORTATION, LLC

                                 By:     /s/  Steven F. Schaab
                                      -----------------------------------------
                                 Name:  S. F. Schaab
                                 Title:  Vice President and Treasurer

                                 POWDER RIVER COAL COMPANY

                                 By:     /s/  Steven F. Schaab
                                      -----------------------------------------
                                 Name:  S. F. Schaab
                                 Title:  Vice President and Treasurer

                                 RIO ESCONDIDO COAL CORP.

                                 By:     /s/  Steven F. Schaab
                                      -----------------------------------------
                                 Name:  S. F. Schaab
                                 Title:  Vice President and Treasurer

                                 SENECA COAL COMPANY

                                 By:  /s/  J. C. Klingl
                                      -----------------------------------------
                                 Name:  J. C. Klingl
                                 Title:  Vice President

                                 SENTRY MINING COMPANY

                                 By:     /s/  Steven F. Schaab
                                      -----------------------------------------
                                 Name:  S. F. Schaab
                                 Title:  Vice President and Treasurer

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                                 SNOWBERRY LAND COMPANY

                                 By:     /s/  Steven F. Schaab
                                      -----------------------------------------
                                 Name:  S. F. Schaab
                                 Title:  Vice President and Treasurer

                                 STERLING SMOKELESS COAL COMPANY

                                 By:  /s/  J. C. Klingl
                                      -----------------------------------------
                                 Name:  J. C. Klingl
                                 Title:  Vice President

                                 THOROUGHBRED, L.L.C.
                                 By:  Peabody Holding Company, Inc.

                                 By:     /s/  Steven F. Schaab
                                      -----------------------------------------
                                 Name:  S. F. Schaab
                                 Title:  Vice President and Treasurer

                                       10<PAGE>
                                                                    EXHIBIT 4.21

                    SIXTH SUPPLEMENTAL SENIOR NOTE INDENTURE

         SIXTH SUPPLEMENTAL SENIOR NOTE INDENTURE (this "Supplemental Senior
Note Indenture"), dated as of February 11, 2002 among THOROUGHBRED GENERATING
COMPANY, LLC, a Delaware limited liability company (the "Guaranteeing
Subsidiary"), a subsidiary of Peabody Energy Corporation (formerly P&L Coal
Holdings Corporation) (or its permitted successor), a Delaware corporation (the
"Company") the Company, the other Senior Note Guarantors (as defined in the
Senior Note Indenture referred to herein) and State Street Bank and Trust
Company, as Senior Note Trustee under the Senior Note Indenture referred to
below (the "Senior Note Trustee").

                                   WITNESSETH

         WHEREAS, the Company has heretofore executed and delivered to the
Senior Note Trustee a Senior Note Indenture (the "Senior Note Indenture"), dated
as of May 18, 1998 providing for the issuance of an aggregate principal amount
of up to $550.0 million of 8-7/8% Senior Notes due 2008 (the "Senior Notes");

         WHEREAS, the Senior Note Indenture provides that under certain
circumstances the Guaranteeing Subsidiary shall execute and deliver to the
Senior Note Trustee a supplemental Senior Note Indenture pursuant to which the
Guaranteeing Subsidiary shall unconditionally guarantee all of the Company's
Obligations under the Senior Notes and the Senior Note Indenture on the terms
and conditions set forth herein (the "Senior Subsidiary Guarantee"); and

         WHEREAS, pursuant to Section 9.01 of the Senior Note Indenture, the
Senior Note Trustee is authorized to execute and deliver this Supplemental
Senior Note Indenture.

         NOW THEREFORE, in consideration of the foregoing and for other good and
valuable consideration, the receipt of which is hereby acknowledged, the
Guaranteeing Subsidiary and the Senior Note Trustee mutually covenant and agree
for the equal and ratable benefit of the Holders of the Senior Notes as follows:

         1.       CAPITALIZED TERMS. Capitalized terms used herein without
definition shall have the meanings assigned to them in the Senior Note
Indenture.

         2.       AGREEMENT TO GUARANTEE. The Guaranteeing Subsidiary hereby
agrees as follows:

                  (a)      Along with all Senior Note Guarantors named in the
                           Senior Note Indenture, to jointly and severally
                           Guarantee to each Holder of a Senior Note
                           authenticated and delivered by the Senior Note
                           Trustee and to the Senior Note Trustee and its
                           successors and assigns, irrespective of the validity
                           and enforceability of the Senior

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                           Note Indenture, the Senior Notes or the obligations
                           of the Company hereunder or thereunder, that:

                           (i)      the principal of and interest on the Senior
                                    Notes will be promptly paid in full when
                                    due, whether at maturity, by acceleration,
                                    redemption or otherwise, and interest on the
                                    overdue principal of and interest on the
                                    Senior Notes, if any, if lawful, and all
                                    other obligations of the Company to the
                                    Holders or the Senior Note Trustee hereunder
                                    or thereunder will be promptly paid in full
                                    or performed, all in accordance with the
                                    terms hereof and thereof; and

                           (ii)     in case of any extension of time of payment
                                    or renewal of any Senior Notes or any of
                                    such other obligations, that same will be
                                    promptly paid in full when due or performed
                                    in accordance with the terms of the
                                    extension or renewal, whether at stated
                                    maturity, by acceleration or otherwise.
                                    Failing payment when due of any amount so
                                    guaranteed or any performance so guaranteed
                                    for whatever reason, the Senior Note
                                    Guarantors shall be jointly and severally
                                    obligated to pay the same immediately.

                  (b)      The obligations hereunder shall be unconditional,
                           irrespective of the validity, regularity or
                           enforceability of the Senior Notes or the Senior Note
                           Indenture, the absence of any action to enforce the
                           same, any waiver or consent by any Holder of the
                           Senior Notes with respect to any provisions hereof or
                           thereof, the recovery of any judgment against the
                           Company, any action to enforce the same or any other
                           circumstance which might otherwise constitute a legal
                           or equitable discharge or defense of a Senior Note
                           Guarantor.

                  (c)      The following is hereby waived: diligence,
                           presentment, demand of payment, filing of claims with
                           a court in the event of insolvency or bankruptcy of
                           the Company, any right to require a proceeding first
                           against the Company, protest, notice and all demands
                           whatsoever.

                  (d)      This Senior Subsidiary Guarantee shall not be
                           discharged except by complete performance of the
                           obligations contained in the Senior Notes and the
                           Senior Note Indenture.

                  (e)      If any Holder or the Senior Note Trustee is required
                           by any court or otherwise to return to the Company,
                           the Senior Note Guarantors, or any custodian, Senior
                           Note Trustee, liquidator or other similar official
                           acting in relation to either the Company or the
                           Senior Note Guarantors, any amount paid by either to
                           the Senior Note Trustee

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                           or such Holder, this Senior Subsidiary Guarantee, to
                           the extent theretofore discharged, shall be
                           reinstated in full force and effect.

                  (f)      The Guaranteeing Subsidiary shall not be entitled to
                           any right of subrogation in relation to the Holders
                           in respect of any obligations guaranteed hereby until
                           payment in full of all obligations guaranteed hereby.

                  (g)      As between the Senior Note Guarantors, on the one
                           hand, and the Holders and the Senior Note Trustee, on
                           the other hand, (x) the maturity of the obligations
                           guaranteed hereby may be accelerated as provided in
                           Article 6 of the Senior Note Indenture for the
                           purposes of this Senior Subsidiary Guarantee,
                           notwithstanding any stay, injunction or other
                           prohibitions preventing such acceleration in respect
                           of the obligations guaranteed hereby, and (y) in the
                           event of any declaration of acceleration of such
                           obligations as provided in Article 6 of the Senior
                           Note Indenture, such obligations (whether or not due
                           and payable) shall forthwith become due and payable
                           by the Senior Note Guarantors for the purpose of this
                           Senior Subsidiary Guarantee.

                  (h)      The Senior Note Guarantors shall have the right to
                           seek contribution from any non-paying Senior Note
                           Guarantor so long as the exercise of such right does
                           not impair the rights of the Holders under the Senior
                           Subsidiary Guarantee.

                  (i)      Pursuant to Section 10.04 of the Senior Note
                           Indenture, after giving effect to any maximum amount
                           and any other contingent and fixed liabilities that
                           are relevant under any applicable Bankruptcy or
                           fraudulent conveyance laws, and after giving effect
                           to any collections from, rights to receive
                           contribution from or payments made by or on behalf of
                           any other Senior Note Guarantor in respect of the
                           obligations of such other Senior Note Guarantor under
                           Article 10 of the Senior Note Indenture shall result
                           in the obligations of such Senior Note Guarantor
                           under its Senior Subsidiary Guarantee not
                           constituting a fraudulent transfer or conveyance.

         3.       EXECUTION AND DELIVERY. Each Guaranteeing Subsidiary agrees
that the Senior Subsidiary Guarantees shall remain in full force and effect
notwithstanding any failure to endorse on each Senior Note a notation of such
Senior Subsidiary Guarantee.

         4.       GUARANTEEING SUBSIDIARY MAY CONSOLIDATE, ETC. ON CERTAIN
TERMS.

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<PAGE>

                  (a)      The Guaranteeing Subsidiary may not consolidate with
                           or merge with or into (whether or not such Senior
                           Note Guarantor is the surviving Person) another
                           corporation, Person or entity whether or not
                           affiliated with such Senior Note Guarantor unless:

                           (i)      subject to Section 10.04 of the Senior Note
                                    Indenture, the Person formed by or surviving
                                    any such consolidation or merger (if other
                                    than a Senior Note Guarantor or the Company)
                                    unconditionally assumes all the obligations
                                    of such Senior Note Guarantor, pursuant to a
                                    supplemental Senior Note Indenture in form
                                    and substance reasonably satisfactory to the
                                    Senior Note Trustee, under the Senior Notes,
                                    the Senior Note Indenture and the Senior
                                    Subsidiary Guarantee on the terms set forth
                                    herein or therein; and

                           (ii)     immediately after giving effect to such
                                    transaction, no Default or Event of Default
                                    exists.

                  (b)      In case of any such consolidation, merger, sale or
                           conveyance and upon the assumption by the successor
                           corporation, by supplemental Senior Note Indenture,
                           executed and delivered to the Senior Note Trustee and
                           satisfactory in form to the Senior Note Trustee, of
                           the Senior Subsidiary Guarantee endorsed upon the
                           Senior Notes and the due and punctual performance of
                           all of the covenants and conditions of the Senior
                           Note Indenture to be performed by the Senior Note
                           Guarantor, such successor corporation shall succeed
                           to and be substituted for the Senior Note Guarantor
                           with the same effect as if it had been named herein
                           as a Senior Note Guarantor. Such successor
                           corporation thereupon may cause to be signed any or
                           all of the Senior Subsidiary Guarantees to be
                           endorsed upon all of the Senior Notes issuable
                           hereunder which theretofore shall not have been
                           signed by the Company and delivered to the Senior
                           Note Trustee. All the Senior Subsidiary Guarantees so
                           issued shall in all respects have the same legal rank
                           and benefit under the Senior Note Indenture as the
                           Senior Subsidiary Guarantees theretofore and
                           thereafter issued in accordance with the terms of the
                           Senior Note Indenture as though all of such Senior
                           Subsidiary Guarantees had been issued at the date of
                           the execution hereof.

                  (c)      Except as set forth in Articles 4 and 5 of the Senior
                           Note Indenture, and notwithstanding clauses (a) and
                           (b) above, nothing contained in the Senior Note
                           Indenture or in any of the Senior Notes shall prevent
                           any consolidation or merger of a Senior Note
                           Guarantor with or into the Company or another Senior
                           Note Guarantor, or shall prevent any sale or
                           conveyance of the property of a Senior

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                           Note Guarantor as an entirety or substantially as an
                           entirety to the Company or another Senior Note
                           Guarantor.

         5.       RELEASES.

                  (a)      In the event of a sale or other disposition of all of
                           the assets of any Senior Note Guarantor, by way of
                           merger, consolidation or otherwise, or a sale or
                           other disposition of all to the capital stock of any
                           Senior Note Guarantor, then such Senior Note
                           Guarantor (in the event of a sale or other
                           disposition, by way of merger, consolidation or
                           otherwise, of all of the capital stock of such Senior
                           Note Guarantor) or the corporation acquiring the
                           property (in the event of a sale or other disposition
                           of all or substantially all of the assets of such
                           Senior Note Guarantor) will be released and relieved
                           of any obligations under its Senior Subsidiary
                           Guarantee; provided that the Net Proceeds of such
                           sale or other disposition are applied in accordance
                           with the applicable provisions of the Senior Note
                           Indenture, including without limitation Section 4.10
                           of the Senior Note Indenture. Upon delivery by the
                           Company to the Senior Note Trustee of an Officer's
                           Certificate and an Opinion of Counsel to the effect
                           that such sale or other disposition was made by the
                           Company in accordance with the provisions of the
                           Senior Note Indenture, including without limitation
                           Section 4.10 of the Senior Note Indenture, the Senior
                           Note Trustee shall execute any documents reasonably
                           required in order to evidence the release of any
                           Senior Note Guarantor from its obligations under its
                           Senior Subsidiary Guarantee.

                  (b)      Any Senior Note Guarantor not released from its
                           obligations under its Senior Subsidiary Guarantee
                           shall remain liable for the full amount of principal
                           of and interest on the Senior Notes and for the other
                           obligations of any Senior Note Guarantor under the
                           Senior Note Indenture as provided in Article 10 of
                           the Senior Note Indenture.

         6.       NO RECOURSE AGAINST OTHERS. No past, present or future
director, officer, employee, incorporator, stockholder or agent of the
Guaranteeing Subsidiary, as such, shall have any liability for any obligations
of the Company or any Guaranteeing Subsidiary under the Senior Notes, any Senior
Subsidiary Guarantees, the Senior Note Indenture or this Supplemental Senior
Note Indenture or for any claim based on, in respect of, or by reason of, such
obligations or their creation. Each Holder of the Senior Notes by accepting a
Senior Note waives and releases all such liability. The waiver and release are
part of the consideration for issuance of the Senior Notes. Such waiver may not
be effective to waive liabilities under the federal securities laws and it is
the view of the Commission that such a waiver is against public policy.

                                       5
<PAGE>
         7.       NEW YORK LAW TO GOVERN. THE INTERNAL LAW OF THE STATE OF NEW
YORK SHALL GOVERN AND BE USED TO CONSTRUE THIS SUPPLEMENTAL SENIOR NOTE
INDENTURE BUT WITHOUT GIVING EFFECT TO APPLICABLE PRINCIPLES OF CONFLICTS OF LAW
TO THE EXTENT THAT THE APPLICATION OF THE LAWS OF ANOTHER JURISDICTION WOULD BE
REQUIRED THEREBY.

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

         9.       EFFECT OF HEADINGS. The Section headings herein are for
convenience only and shall not affect the construction hereof.

         10.      THE SENIOR NOTE TRUSTEE. The Senior Note Trustee shall not be
responsible in any manner whatsoever for or in respect of the validity or
sufficiency of this Supplemental Senior Note Indenture or for or in respect of
the recitals contained herein, all of which recitals are made solely by the
Guaranteeing Subsidiary and the Company.

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

                                       THOROUGHBRED GENERATING COMPANY, LLC

                                             By:  /s/ Steven F. Schaab
                                                ---------------------------
                                             Name:  S. F. Schaab
                                             Title:  Vice President & Treasurer

                                       6

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