six-and-four, n.² — Green’s Dictionary of Slang

six-and-four n.²

(drugs) heroin that has been adulterated and weakened by mixing one part pure heroin to six or four parts sugar.

1965		Everygreen Rev. IX 15/1: By six and four I mean cutting it one spoon of pure heroin and six or four of sugar.
1999		M. Booth Opium 86: Heavily cut heroin is known as ‘six and four’.