Green’s Dictionary of Slang

1. an act of deception.

1896		F. Hutcheson Barkeep Stories 183: ‘He trun a four-flush into a guy de odder night, dough, dat’d ’a done yer heart good to got a peek at’.
1908		Chicago Trib. 27 Sept. in Fleming Unforgettable Season (1981) 259: The New York National League club is engaged in one of its familiar fourflushes [...] Broadway, from Times Square to the Battery, is the native heath of the fourflush.

2. see four-flusher n.