Exhibit 4.18

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MARKWEST
ENERGY PARTNERS, L.P.

MARKWEST
ENERGY FINANCE CORPORATION

and

the
Subsidiary Guarantors named herein



81/2%
SENIOR NOTES DUE 2016





FOURTH
SUPPLEMENTAL INDENTURE

DATED
AS OF SEPTEMBER 15, 2008



WELLS
FARGO BANK, NATIONAL ASSOCIATION,

Trustee



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This FOURTH SUPPLEMENTAL INDENTURE, dated as of September 15, 2008 is among MarkWest Energy Partners, L.P., a Delaware limited partnership (the "Partnership"), MarkWest
Energy Finance Corporation, a Delaware corporation ("MarkWest Finance" and, together with the Partnership, the "Issuers"), each of the parties identified under the caption "Subsidiary Guarantors" on
the signature page hereto (the "Subsidiary Guarantors") and Wells Fargo Bank, National Association, a national banking association, as Trustee.

RECITALS

WHEREAS,
the Issuers, the initial Subsidiary Guarantors and the Trustee entered into an Indenture, dated as of July 6, 2006 (as amended and
supplemented, the "Indenture"), pursuant to which the Issuers have issued $200,000,000 in principal amount of 81/2% Senior Notes due 2016 (the "Notes");

WHEREAS,
Section 9.01(d) of the Indenture provides that the Issuers, the Subsidiary Guarantors and the Trustee may amend or supplement the Indenture in order to add a Subsidiary
Guarantor pursuant to Section 4.13 or 5.01(c) thereof, without the consent of the Holders of the Notes; and

WHEREAS,
all acts and things prescribed by the Indenture, by law and by the Certificate of Incorporation and the Bylaws (or comparable constituent documents) of the Issuers, of the
Subsidiary Guarantors and of the Trustee necessary to make this Supplemental Indenture a valid instrument legally binding on the Issuers, the Subsidiary Guarantors and the Trustee, in accordance with
its terms, have been duly done and performed;

NOW,
THEREFORE, to comply with the provisions of the Indenture and in consideration of the above premises, the Issuers, the Subsidiary Guarantors and the Trustee covenant and agree for
the equal and proportionate benefit of the respective Holders of the Notes as follows:

ARTICLE 1

Section 1.01.    This
Supplemental Indenture is supplemental to the Indenture and does and shall be deemed to form a part of, and shall be construed in connection
with and as part of, the Indenture for any and all purposes.

Section 1.02.    This
Supplemental Indenture shall become effective immediately upon its execution and delivery by each of the Issuers, the Subsidiary Guarantors and the
Trustee.

ARTICLE 2

From
this date, in accordance with Section 4.13 or 5.01(c) and by executing this Supplemental Indenture, the Guarantors whose signatures appear below
are subject to the provisions of the Indenture to the extent provided for in Article 10 thereunder.

ARTICLE 3

Section 3.01.    Except
as specifically modified herein, the Indenture and the Notes are in all respects ratified and confirmed(mutatis
mutandis)and shall remain in full force and effect in accordance with their terms with all capitalized terms used herein without definition having the same respective meanings
ascribed to them as in the Indenture.

Section 3.02.    Except
as otherwise expressly provided herein, no duties, responsibilities or liabilities are assumed, or shall be construed to be assumed, by the
Trustee by reason of this Supplemental Indenture. This Supplemental Indenture is executed and accepted by the Trustee subject to all the
terms and conditions set forth in the Indenture with the same force and effect as if those terms and conditions were repeated at length herein and made applicable to the Trustee with respect hereto.

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Section 3.03.    THIS
SUPPLEMENTAL INDENTURE SHALL BE GOVERNED BY, AND CONSTRUED IN ACCORDANCE WITH, THE LAWS OF THE STATE OF NEW YORK.

Section 3.04.    The
parties may sign any number of copies of this Supplemental Indenture. Each signed copy shall be an original, but all of such executed copies
together shall represent the same agreement.

[NEXT
PAGE IS SIGNATURE PAGE]

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

Issuers:MARKWEST ENERGY PARTNERS, L.P.By:

MarkWest Energy GP, L.L.C., its general partnerBy:

/s/ Andrew L. SchroederName:

Andrew L. SchroederTitle:

Vice President Finance, Treasurer and
Assistant SecretaryMARKWEST ENERGY FINANCE CORPORATIONBy:

/s/ Andrew L. SchroederName:

Andrew L. SchroederTitle:

Vice President Finance, Treasurer and
Assistant Secretary


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Subsidiary Guarantors:MARKWEST ENERGY GP, L.L.C.By:

/s/ Andrew L. SchroederName:

Andrew L. SchroederTitle:

Vice President Finance, Treasurer and
Assistant SecretaryMARKWEST HYDROCARBON, INC.By:

/s/ Andrew L. SchroederName:

Andrew L. SchroederTitle:

Vice President Finance, Treasurer and
Assistant SecretaryMASON PIPELINE LIMITED LIABILITY COMPANYBy:

MarkWest Hydrocarbon, Inc., its Sole MemberBy:

/s/ Andrew L. SchroederName:

Andrew L. SchroederTitle:

Vice President Finance, Treasurer and
Assistant Secretary


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MARKWEST ENERGY OPERATING COMPANY,
    L.L.C.By:

MarkWest Energy Partners, L.P., its
Managing MemberBy:

MarkWest Energy GP, L.L.C., its General
PartnerBy:

/s/ Andrew L. SchroederName:

Andrew L. SchroederTitle:

Vice President Finance, Treasurer and
Assistant SecretaryBASIN PIPELINE L.L.C.
WEST SHORE PROCESSING COMPANY, L.L.C.By:

MarkWest Energy Operating Company,
L.L.C., its Sole Member and ManagerBy:

MarkWest Energy Partners, L.P., its
Managing MemberBy:

MarkWest Energy GP, L.L.C., its General
PartnerBy:

/s/ Andrew L. SchroederName:

Andrew L. SchroederTitle:

Vice President Finance, Treasurer and
Assistant Secretary


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MARKWEST BLACKHAWK, L.L.C.
MARKWEST ENERGY APPALACHIA, L.L.C.
MARKWEST ENERGY EAST TEXAS GAS
    COMPANY, L.L.C.
MARKWEST GAS MARKETING, L.L.C.
MARKWEST GAS SERVICES, L.L.C.
MARKWEST JAVELINA COMPANY, L.L.C.
MARKWEST JAVELINA PIPLELINE
    COMPANY, L.L.C.
MARKWEST LIBERTY GAS GATHERING,
    L.L.C.
MARKWEST MARKETING, L.L.C.
MARKWEST NEW MEXICO, L.L.C.
MARKWEST PINNACLE, L.L.C.
MARKWEST PIONEER, L.L.C.
MARKWEST PIPELINE COMPANY, L.L.C.
MARKWEST PNG UTILITY, L.L.C.
MARKWEST POWER TEX, L.L.C.
MARKWEST TEXAS PNG UTILITY, L.L.C.By:

MarkWest Energy Operating Company,
L.L.C., its Sole MemberBy:

MarkWest Energy Partners, L.P., its
Managing MemberBy:

MarkWest Energy GP, L.L.C., its General
PartnerBy:

/s/ Andrew L. SchroederName:

Andrew L. SchroederTitle:

Vice President Finance, Treasurer and
Assistant Secretary


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MARKWEST MICHIGAN PIPELINE COMPANY, L.L.C.MARKWEST OKLAHOMA GAS COMPANY, L.L.C.By:

MarkWest Energy Operating Company,
L.L.C., its Managing MemberBy:

MarkWest Energy Partners, L.P., its
Managing MemberBy:

MarkWest Energy GP, L.L.C., its General
PartnerBy:

/s/ Andrew L. SchroederName:

Andrew L. SchroederTitle:

Vice President Finance, Treasurer and
Assistant SecretaryMATREX, L.L.C.By:

Basin Pipeline L.L.C., its Sole Member and
ManagerBy:

MarkWest Energy Operating Company,
L.L.C., its Sole Member and ManagerBy:

MarkWest Energy Partners, L.P., its
Managing MemberBy:

MarkWest Energy GP, L.L.C., its General
PartnerBy:

/s/ Andrew L. SchroederName:

Andrew L. SchroederTitle:

Vice President Finance, Treasurer and
Assistant Secretary


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MARKWEST MCALESTER, L.L.C.By:

MarkWest Oklahoma Gas Company, L.L.C.,
its Sole MemberBy:

MarkWest Energy Operating Company, L.L.C.,
its Managing MemberBy:

MarkWest Energy Partners, L.P., its
Managing MemberBy:

MarkWest Energy GP, L.L.C., its General
PartnerBy:

/s/ Andrew L. SchroederName:

Andrew L. SchroederTitle:

Vice President Finance, Treasurer and
Assistant Secretary


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Trustee:WELLS FARGO BANK, NATIONAL
ASSOCIATION, as TrusteeBy:

/s/ Patrick T. GiordanoName:

Patrick T. GiordanoTitle:

Vice President


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