1. in context of alcohol.
(a) [mid-19C–1910s] fourpenny-worth of a given drink, as sold in a public house.
(b) (also four ounce) a four ounce beer glass; a serving of beer in such a glass.
2. [2010s] (UK Black/gang) a four-wheel drive vehicle.
3. [2010s] (UK Black/gang) a .44 pistol.
SE in slang uses
[mid-19C] a cheap hat.
[1940s] (US black) the fifth day of the working week, i.e. payday, which is Friday.
[1940s] (W.I.) a courthouse.
[1930s] a sandwich.
[1990s+] a four-wheel-drive vehicle, usu. a form of Jeep, popular among drug dealers, rappers and their fans.
see four-flusher n.
[mid-19C–1920s] conforming with, agreeing, fitting.