Green’s Dictionary of Slang

(US) useless, inferior, weak.

1944		‘F. Bonnamy’ A Rope of Sand (1947) 119: Like many another outwardly impressive specimen, he’s Four-F.
1949		(con. 1943–5) A. Murphy To Hell and Back (1950) 100: Those 4-F, draft-dodging bastards.
1967		(con. 1940s) M. Dibner Admiral (1968) 257: We’ll close it so tight you’ll never in your frigging four-eff life be able to open her again. [Ibid.] 258: You fat four-eff bastard.
1973		G. Swarthout Luck and Pluck 79: If you hadn’t been Four-F or gone to Canada or been a CO or tricked Nixon some way, I wouldn’t give you the time of day.
1976		N. Thornburg Cutter and Bone (2001) 238: A guilt payment. Because he’s loaded. Because he was 4F.
2012		E. Kurtz ‘In the Neighborhood’ in ThugLit Dec. [ebook] Junior was no vet—he was 4-F and would never say why.