The Secret: The tube with the plug inside will retain a coin when it is turned upside down. That same coin will release when it is tapped on the side (i.e. If 4 coins are put in and dumped out, only 3 will exit; one will be held up in the underside of the tube).
Preparation: Hide one of the 5 coins in your left hand.
Performance: Pass out for inspection the rings, plug, and 4 coins. Remember, your spectator should never know you have a 5th coin.