knock on the door, a, n. — Green’s Dictionary of Slang

knock on the door, a n.

also knock at the door, a

[rhy. sl.]

(bingo) the number four.

1972		(con. 1940s) N. Conway Bloods 115: The caller had sung out his litany of Legs Eleven, Kelly’s Eye, Knock on the Door, Clickety-Clicks.
1981		P. Wright Cockney Dialect and Sl. 109: 4 = knock at the door.
1992		R. Puxley Cockney Rabbit.
2002		B. Kirkpatrick Wicked Cockney Rhy. Sl. 46: knock at the door four.