Jimmy Carter's Peanut Butter Pie

6 ounces cream cheese
3/4 cup confectioners' sugar
1/2 cup peanut butter
2 tablespoons milk
1 (8 ounce) carton Cool Whip
1 baked graham cracker crust

Beat cream cheese and confectioners' sugar together. Add peanut butter and milk. Fold in Cool Whip. Pour into baked graham cracker crust and chill for several hours. Serve with additional whipped topping, it desired.