Green’s Dictionary of Slang

(US) to cheat or bluff; thus four-flushing n.

1896		Ade Artie (1963) 26: Not that boy. He was four-flushin’, I know the brand.
1905		H. Green Actors’ Boarding House (1906) 263: If you’re four-flushin’ say so, ’cause I won’t stand no monkey business.
1916		O.O. McIntyre New York Day by Day 24 Apr. [synd. col.] He is a plain blunt sould whose pride it is never to fourflush, misrepresent or be unnatural.
1921		O.O. McIntyre New York Day by Day 7 Feb. [synd. col.] Folks who like friends to believe they stand solid with theatrical producers [...] buy a puncher to complete the bit of four flushing.
1934		O. Strange Sudden 228: Burdette is four-flushin’, Purdie [...] Yu can call his bluff.
1939		P. Cheyney Don’t Get Me Wrong (1956) 7: I would rather four-flush a team of wild alligators outa their lunch-pail.
1943		Baker ‘Influence of Amer. Sl. on Aus.’ in AS XVIII:4 255: A number of synonyms for his own slang words, such as bloke for ‘guy,’ skiting for ‘four flushing,’ nark for ‘crab’.