In poker, five cards from the same suit is called a flush and is very valuable.
The highest possible poker hand is a royal flush, or five cards from ten to ace all from the same suit.
However, four cards from the same suit, or a four flush, is nearly worthless.
If a person continues to play with such a hand they are bluffing, or hiding the truth.
And that gave us the expression “four flusher” for a shifty person not to be trusted or believed.