In a mixing 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" pan.
In a metal bowl over simmering water, or in a microwave-safe bowl in the microwave, melt chocolate chips with 4 tbsp peanut butter, stirring occasionally until smooth. Spread over prepared crust. Refrigerate for at least one hour before cutting into squares and serving.