Green’s Dictionary of Slang

(US) cheating, crooked, untrustworthy.

1899		Ade Fables in Sl. (1902) 12: She always saw the same old line of Four-Flush drummers from Chicago and St. Louis, smoking Horrid Cigars.
1901		‘Hugh McHugh’ John Henry 11: The four-flush call-down makes you back-pedal.
1906		S. Ford Shorty McCabe 87: It’s me to go back to trainin’ four flush comers again.
1920		H.C. Witwer Kid Scanlon 186: Why don’t you can that four-flush thing?