Bake the cake according to package instructions. Slice each layer into 2 layers (this is easier if partially frozen).
Combine the sugar, sour cream, and coconut together until well mixed. Remove 1 cup of this mixture and set aside.
Spread 1/3 of the remaining sugar/coconut/sour cream mixture on each of the 3 layers, leaving about 1/2″ around the edges. Top with the 4th cake layer.
Add the Cool Whip to the 1 cup of reserved sugar/coconut/sour cream mixture that you previously set aside and gently fold it all together until well mixed.
Ice the top and sides of the cake with the above icing. Cover and put in refrigerator and leave for 3 days. Enjoy!