Abstract. The "five-card trick" invented by Boer allows Alice and Bob to securely compute the AND function of their secret inputs using five cards-three black cards and two red cards-with identical backs. This paper shows that such a secure computation can be done with only four cards. Specifically, we give a protocol to achieve a secure computation of AND using only four cards-two black and two red. Our protocol is optimal in the sense that the number of required cards is minimum.