Prepare cake according to package direction for a 9x13 cake. Once cake comes out of the oven,allow to cool completely. Then with a wooden spoon handle poke holes in the cake. you want the holes to be fairly big so that the pudding has plenty of room to get down in the cake.Be sure to poke right down to the bottom of the cake.
In a bowl whisk together the pudding with the 4 cups of milk. Stir until all lumps are gone. Let pudding set for about 5 minutes so it has begun to thicken but not fully set. It should still be pourable. Pour the pudding over the cake,making sure to get it into the holes as much as possible.
Put the cake into the fridge to set about 2 or 3 hrs. Spread on cool whip and sprinkle crushed vanilla wafers on top. SO YUMMY!!!