Anyone who plays poker knows five cards in a single suit is called a flush. What happens if you have only four of that suit? A whole lot of nothing, which gives us the word four-flusher, a person bluffing at cards, or someone who seems promising, but isn't.
Use four-flusher as a synonym for conman, trickster, or fake. You won't hear it used very often these days, but hop in a time machine (a four-flusher will sell you a ticket), travel back to the Old West, kick open the bat-wing doors of the local saloon, and you might hear four-flusher slung across a card table as a brawl gets underway. You might even hear it used as a verb, as in, "You better stop four-flushing, or I'll run you out of this town on a pole."