Prepare cake according to package directions. Bake as directed for a 9x13 inch pan. Allow to cool.
Prepare the strawberry gelatin according to package instructions, using the 4 cups of water. Pour over cooled cake while still in liquid form; the cake will absorb it. Chill in refrigerator for 2 hours, or until gelatin is set.
Spread whipped topping over cake and top with sliced strawberries.