The spectator close again his hand, and the magician touch again his hand with the coin. When he open his hand, the spectator discover now 3 sponges !
Finally the spectator close his hand for the last time and the magician ask him if he is waiting to see 4 sponges...He touches the hand with the coin and request the spectator to open his hand. There is an enormous 4 in his hand !
Easy to realize.