Green’s Dictionary of Slang

(US black) a .45 handgun.

1990		Kool G Rap & DJ Polo ‘Money In The Bank’ 🎵 I grab the mic, and I load it like a long four-fifth.
2003		Neptunes ‘Rock N’ Roll’ 🎵 The fists get to swingin’ and the four-fifths spark.
2011		Heavy Metal Kings ‘Santa Sangre’ 🎵 I’ma let this four-fifth bark like it’s a pitbull.