In a medium bowl, mix together the butter or margarine, graham cracker crumbs, confectioners' sugar, and 1 cup peanut butter until well blended. Press evenly into the bottom of an ungreased 9x13 inch pan. In a metal bowl over simmering water, or in a microwave-safe bowl in the microwave, melt the chocolate chips with the 4 tablespoons peanut butter, stirring occasionally until smooth. Spread over the prepared crust. Refrigerate for at least one hour before cutting into squares.