Acknowledgement of Release of Subsidiary Guarantor by Wells Fargo Bank, N.A. (MarkWest Energy Indentures)

Exhibit 4.1

ACKNOWLEDGEMENT OF RELEASE OF
SUBSIDIARY GUARANTOR

May 1, 2009

        WELLS FARGO BANK, NATIONAL ASSOCIATION, acting as Trustee under each of the three Indentures among MarkWest Energy Partners, L.P., a Delaware limited partnership and MarkWest Energy Finance Corporation, a Delaware corporation, the Subsidiary Guarantors party thereto and the Trustee, dated as of October 25, 2004, July 6, 2006 and April 15, 2008, in each case as amended and supplemented to the date hereof (collectively, the "Indentures"), does hereby acknowledge that MarkWest Pioneer, L.L.C., a Delaware limited liability company, has been released from its obligations under its Guarantees (including any guarantee notation endorsed on any Note issued under the Indentures) and the Indentures in accordance with the provisions of 10.05(v) of each of the Indentures and has no continuing obligation or liability under its Guarantees or the Indentures whether arising before or after such releases.

        All capitalized terms used herein and not defined herein have the meanings attributed to them in the Indentures.

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        IN WITNESS WHEREOF, the undersigned has executed this acknowledgment as of the date first written above.

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Exhibit 4.1