how the f did this magician do this trick

[Roswell]

Last night I was out to dinner and a magician came up and started doing various card and coin tricks. This guy was pretty amazing. The most incredible trick he did was this: he gave me 4 silver dollars, had me close my palm, and said "one of them will appear in my hand." He then opens his (previously empty) hand and there's a single silver dollar there. I then open my palm and now there's only THREE silver dollars there! WTF!!

We asked him about the trick later and he said there was a common trick in which the magician has 4 coins, closes his palm, and opens it to reveal only 3 coins. However, his modification was that an audience member holds the 4 coins, and one disappears. He said "There's only one thing people can do to screw it up, but they almost never do it."

So how did he do it??

btw, I asked if he knew about Antonio and he did not. that's not rocks and rings.