1 gallon of milk is equal to 16 cups of milk (1 gallon = 4 quarts = 16 cups).
Mario poured 4 cups of milk for each of the 4 people, which is a total of 16 cups of milk.
So the amount of milk left is:
16 cups (total) - 16 cups (poured) = 0 cups
Therefore, there is no milk left.