Green’s Dictionary of Slang

1. (UK society) an unpleasant person; the four letters are perhaps s-h-i-t (as suggested by Manchon, Le Slang, 1923) or c-u-n-t (cf. five-letter woman under five adj.).

1953		C. Lederer Gentlemen Prefer Blondes [film script] ‘I’m a four-letter man!’ ‘Well, I wouldn’t brag about it.’.
1977		Maledicta 1 (Summer) 13: A person that you don’t like is often described in terms of various bodily parts or excretions: he is [...] a shit (and therefore a Four-Letter Man).
1988		D. White in Costello Mask of Treachery 565: I thought he [i.e. the spy Anthony Blunt] was a four-letter man.
1993		in A. Caesar Taking It Like a Man 202: If you’d been anyone else you’d have thought me a first-class four-letter man for changing the dedication like that.
2003		‘Amer. vs British Eng.’ on University of Tampere FAST Area Studies Program 🌐 ‘A five-letter woman married to a four-letter man.’.

2. (US) a male homosexual: both as sense 1 and as h-o-m-o.

1949		Monteleone Criminal Sl. (rev. edn) 90: four letter man A male homosexual.
1972		B. Rodgers Queens’ Vernacular 109: male homosexual [...] four-letter man (H-O-M-O).