Show the coins to spectators and make sure that they acknowledge that there are four coins.
Close your hand and crush the foil. If you can, into a tiny ball.
When you open your hand again, display three coins. The piece of foil is hidden under the other coins.
Note that you may have to experiment with the foil to get one that resembles the other coins. In the photographs here, the foil coin looks quite different from the other coins, but in real life, more closely resembled the other coins.
Also, you'll want to experiment with the placement of the foil relative to the other coins. The key here is to situate the fake foil coin so it's easy to crush when you close your hand.
