If you look carefully, he's taking out the bottom card as he flips the top card. That way, the bottom card is flipped over to the top of the top card. Then when the top card is discarded, the original top card is still there. No matter what he does, he takes the bottom card and slides it to the top. That's why he's using only 4 cards, cuz 4 cards allows the bottom card to be slipped over without being noticed.