For this case we have to know the number of pieces in which each cupcake must be cut, so that each friend has four pieces of cupcake. So:
IF they are 16 friends and each one must have four pieces of cupcake, we have the following multiplication:
64 pieces of cupcakes are needed.
Now, if we only have 4 cupcakes, we should know how many pieces each should be chopped to get 64 pieces of cupcake. So:
Let "x" be the variable that defines the amount of pieces in which each cupcake should be chopped:

By clearing "x" we have:

So, each cupcake should be cut into 16 pieces so that each friend has 4 pieces of cupcake
Answer:
Each cupcake should be cut into 16 pieces so that each friend has 4 pieces of cupcake