Document:

EX-10.4

Exhibit 10.4

PROMISSORY NOTE

March 15, 2006

$4,160,000.00

FOR VALUE RECEIVED, the undersigned, HyperFeed Technologies, Inc., a Delaware corporation
(“Borrower”), promises to pay to the order of PICO Holdings, Inc., a California corporation
(“Lender”), the principal sum of Four Million One Hundred Sixty Thousand dollars and no cents
($4,160,000.00), together with the interest thereon at the rate of seven percent (7%) per annum,
commencing on the date hereof and continuing until the principal sum is paid in full. Interest
shall be computed on the basis of a 365-day year.

The parties acknowledge that Borrower borrowed the principal sum of $3,310,000 from Lender in
a Promissory Note dated February 15, 2006.

The parties agree that an additional principal sum of $850,000 shall be borrowed from Lender in
this Promissory Note dated March 15, 2006 and that the principal sum borrowed by Borrower from
Lender in the February 15, 2006 Promissory Note and this March 15, 2006 Promissory Note totals
$4,160,000. The parties also agree that the February 15, 2006 Promissory Note totaling $3,310,000
is hereby cancelled.

The entire unpaid principal balance of, and all interest under, this Promissory Note shall be due
and payable not later than April 30, 2006. The indebtness evidenced hereby may be prepaid in whole
or part at any time without penalty. Any payment or prepayment shall be applied first to interest
and second to the principal due and payable hereunder.

HyperFeed Technologies, Inc. covenants that, if suit be brought to enforce the payment of the
principal or interest under this Promissory Note, it will pay PICO Holdings, Inc. such further
amount as shall be reasonable to cover the cost and expense of collection including reasonable
attorney fees.

HYPERFEED TECHNOLOGIES, INC.

/s/ Gemma Lahera          

By: Gemma R. Lahera

Title: Principal Accounting Officer & TreasurerEX-4.1

Exhibit 4.1

THIRD SUPPLEMENTAL INDENTURE FOR

ADDITIONAL SUBSIDIARY GUARANTEES

Third Supplemental Indenture (this “Supplemental Indenture for Additional
Guarantees”), dated as of March 14, 2006, among the guaranteeing subsidiaries listed on
Schedule A hereto (the “Guaranteeing Subsidiaries”), each an indirect subsidiary of NRG
Energy, Inc., a Delaware corporation (the “Company”), the Company, the other Guarantors (as defined
in the Indenture referred to herein) and Law Debenture Trust Company of New York, as trustee under
the indentures 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 (the
“Base Indenture”), dated as of February 2, 2006, between the Company and the Trustee, as amended by
a first supplemental indenture (the “First Supplemental Indenture” and, together with the Base
Indenture, the “Indenture”), dated as of February 2, 2006, among the Company, the Guarantors named
therein and the Trustee, providing for the original issuance of an aggregate principal amount of
$1,200 million of 7.250% Senior Notes due 2014 (the “Initial Notes”), and, subject to the terms of
the Indenture, future unlimited issuances of 7.250% Senior Notes due 2014 (the “Additional Notes,”
and together with the Initial Notes, the “Notes”);

WHEREAS, the Indenture provides that under certain circumstances the Guaranteeing Subsidiaries
shall execute and deliver to the Trustee a supplemental indenture pursuant to which the
Guaranteeing Subsidiaries shall unconditionally guarantee all of the Company’s Obligations under
the Notes and the Indenture (the “Additional Guarantees”); and

WHEREAS, pursuant to Section 4.17 of the First Supplemental Indenture, the Trustee, the
Company and the other Guarantors are authorized and required to execute and deliver this
Supplemental Indenture for Additional Guarantees.

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

1. Capitalized Terms. Unless otherwise defined in this Supplemental Indenture for Additional
Guarantees, capitalized terms used herein without definition shall have the meanings assigned to
them in the Indenture.

2. Agreement to be Bound; Guarantee. Each of the Guaranteeing Subsidiaries hereby becomes a
party to the First Supplemental Indenture as a Guarantor and as such will have all of the rights
and be subject to all of the Obligations and agreements of a Guarantor under the Indenture. Each
of the Guaranteeing Subsidiaries hereby agrees to be bound by all of the provisions of the First
Supplemental Indenture applicable to a Guarantor and to perform all of the Obligations and
agreements of a Guarantor under the First Supplemental Indenture. In furtherance of the foregoing,
each of the Guaranteeing Subsidiaries shall be deemed a Guarantor for purposes of Article 10 of the
First Supplemental Indenture, including, without limitation, Section 10.02 thereof.

3. NEW YORK LAW TO GOVERN. THE INTERNAL LAW OF THE STATE OF NEW YORK SHALL GOVERN AND BE USED
TO CONSTRUE THIS SUPPLEMENTAL INDENTURE FOR ADDITIONAL GUARANTEES 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.

4. Counterparts. The parties may sign any number of copies of this Supplemental Indenture for
Additional Guarantees. 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 Trustee. The Trustee shall not be responsible in any manner whatsoever for or in
respect of the validity or sufficiency of this Supplemental Indenture for Additional Guarantees or
for or in respect of the recitals contained herein, all of which recitals are made solely by the
Guaranteeing Subsidiaries and the Company.

7. Ratification of Indenture; Supplemental Indenture for Additional Guarantees 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 for Additional Guarantees shall form a part of the Indenture
for all purposes, and every Holder of Notes heretofore or hereafter authenticated and delivered
shall by bound hereby.

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

Guaranteeing Subsidiaries:

NRG BLOCKER I LP

NRG BLOCKER II LP

NRG BLOCKER III INC.

NRG BLOCKER IV INC.

NRG BLOCKER V INC.

NRG BLOCKER VI INC.

NRG BLOCKER VII INC.

TEXAS GENCO LLC

TEXAS GENCO FINANCING CORP.

TEXAS GENCO GP, LLC

NEW GENCO GP, LLC

TEXAS GENCO OPERATING SERVICES LLC

TEXAS GENCO HOLDINGS, INC.

By:  /s/ George P. Schaefer

Name: George P. Schaefer

Title: Vice President

GCP FUNDING COMPANY, LLC

NEW GENCO LP, LLC

TEXAS GENCO LP, LLC

By:  /s/ Susan T. Dubb

Name: Susan T. Dubb

Title: Treasurer and Secretary

TEXAS GENCO II, LP

TEXAS GENCO SERVICES, LP

By: New Genco GP, LLC, its General Partner

By:  /s/ George P. Schaefer

Name: George P. Schaefer

Title: Vice President and Assistant Treasurer

TEXAS GENCO, LP

By: Texas Genco GP, LLC, its General Partner

By:  /s/ George P. Schaefer

Name: George P. Schaefer

Title: Vice President and Assistant Treasurer

Issuer:

NRG ENERGY, INC.

By:  /s/ George P. Schaefer

Name: George P. Schaefer

Title: Vice President and Treasurer

Existing Guarantors:

ARTHUR KILL POWER LLC

ASTORIA GAS TURBINE POWER LLC

BERRIANS I GAS TURBINE POWER LLC

BIG CAJUN II UNIT 4 LLC

CAPISTRANO COGENERATION COMPANY

CHICKAHOMINY RIVER ENERGY CORP.

COMMONWEALTH ATLANTIC POWER LLC

CONEMAUGH POWER LLC

CONNECTICUT JET POWER LLC

DEVON POWER LLC

DUNKIRK POWER LLC

EASTERN SIERRA ENERGY COMPANY

HANOVER ENERGY COMPANY

HUNTLEY POWER LLC

INDIAN RIVER OPERATIONS INC.

INDIAN RIVER POWER LLC

JAMES RIVER POWER LLC

KAUFMAN COGEN LP

KEYSTONE POWER LLC

LOUISIANA GENERATING LLC

MIDDLETOWN POWER LLC

MONTVILLE POWER LLC

NEO CALIFORNIA POWER LLC

NEO CHESTER-GEN LLC

NEO CORPORATION

NEO FREEHOLD-GEN LLC

NEO LANDFILL GAS HOLDINGS INC.

NEO POWER SERVICES INC.

NORWALK POWER LLC

NRG AFFILIATE SERVICES INC.

NRG ARTHUR KILL OPERATIONS INC.

NRG ASIA-PACIFIC, LTD.

NRG ASTORIA GAS TURBINE OPERATIONS, INC.

NRG BAYOU COVE LLC

NRG CABRILLO POWER OPERATIONS INC.

NRG CADILLAC OPERATIONS INC.

NRG CALIFORNIA PEAKER OPERATIONS LLC

NRG CONNECTICUT AFFILIATE SERVICES INC.

NRG DEVON OPERATIONS INC.

NRG DUNKIRK OPERATIONS INC.

NRG EL SEGUNDO OPERATIONS INC.

NRG HUNTLEY OPERATIONS INC.

NRG INTERNATIONAL LLC

NRG KAUFMAN LLC

NRG MESQUITE LLC

NRG MIDATLANTIC AFFILIATE SERVICES INC.

NRG MIDDLETOWN OPERATIONS INC.

NRG MONTVILLE OPERATIONS INC.

NRG NEW JERSEY ENERGY SALES LLC

NRG NEW ROADS HOLDINGS LLC

NRG NORTH CENTRAL OPERATIONS INC.

NRG NORTHEAST AFFILIATE SERVICES INC.

NRG NORWALK HARBOR OPERATIONS INC.

NRG OPERATING SERVICES, INC.

NRG OSWEGO HARBOR POWER OPERATIONS INC.

NRG POWER MARKETING INC.

NRG ROCKY ROAD LLC

NRG SAGUARO OPERATIONS INC.

NRG SOUTH CENTRAL AFFILIATE SERVICES INC.

NRG SOUTH CENTRAL GENERATING LLC

NRG SOUTH CENTRAL OPERATIONS INC.

NRG WEST COAST LLC

NRG WESTERN AFFILIATE SERVICES INC.

OSWEGO HARBOR POWER LLC

SAGUARO POWER LLC

SOMERSET OPERATIONS INC.

SOMERSET POWER LLC

VIENNA OPERATIONS INC.

VIENNA POWER LLC

By:  /s/ George P. Schaefer

Name: George P. Schaefer

Title: Treasurer

Attest:

/s/ Tanuja M. Dehne

Name: Tanuja M. Dehne

Title: Corporate Secretary

LAW DEBENTURE TRUST COMPANY OF NEW YORK, as Trustee

By:  /s/ Adam Berman

Name: Adam Berman

Title: Vice President

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Schedule A

SCHEDULE OF GUARANTEEING SUBSIDIARIES

	 	1.	 	Texas Genco LLC, a Delaware limited liability company

	 	2.	 	Texas Genco Financing Corp., a Delaware corporation

	 	3.	 	Texas Genco GP, LLC, a Texas limited liability company

	 	4.	 	New Genco GP, LLC, a Delaware limited liability company

	 	5.	 	New Genco LP, LLC, a Delaware limited liability company

	 	6.	 	Texas Genco Operating Services LLC, a Delaware limited liability company

	 	7.	 	Texas Genco Holdings, Inc., a Texas corporation

	 	8.	 	Texas Genco II, LP, a Texas limited partnership

	 	9.	 	Texas Genco Services, LP, a Texas limited partnership

	 	10.	 	GCP Funding Company, LLC, a Delaware limited liability company

	 	11.	 	Texas Genco LP, LLC, a Delaware limited liability company

	 	12.	 	Texas Genco, LP, a Texas limited partnership

	 	13.	 	NRG Blocker I LP, a Delaware limited partnership

	 	14.	 	NRG Blocker II LP, a Delaware limited partnership

	 	15.	 	NRG Blocker III Inc., a Delaware corporation

	 	16.	 	NRG Blocker IV Inc., a Delaware corporation

	 	17.	 	NRG Blocker V Inc., a Delaware corporation

	 	18.	 	NRG Blocker VI Inc., a Delaware corporation

	 	19.	 	NRG Blocker VII Inc., a Delaware corporation

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