EXHIBIT 4.A EIGHTH SUPPLEMENTAL INDENTURE among EL PASO EXPLORATION & PRODUCTION COMPANY, as Issuer, MBOW FOUR STAR CORPORATION, EL PASO EXPLORATION & PRODUCTION MANAGEMENT, INC., EL PASO E&P HOLDINGS, INC., EL PASO E&P COMPANY, L.P., EL PASO PRODUCTION RESALE COMPANY, EL PASO ENERGY OIL TRANSMISSION, L.L.C., EL PASO PRODUCTION OIL & GAS GATHERING, L.P., EL PASO E&P INTERNATIONAL HOLDING COMPANY, EL PASO PREFERRED HOLDINGS COMPANY, EL PASO E&P FINANCE COMPANY, L.L.C., EL PASO SOUTH TEXAS E&P COMPANY, L.L.C., and EL PASO E&P ZAPATA, L.P., as Subsidiary Guarantors, and WILMINGTON TRUST COMPANY, as Trustee June 13, 2007 7