Term used in hip-hop culture to refer to rappers who write/freestyle only focusing on the fourth bar of each quatrain, leaving the previous 3 bars with nothing interesting to say. DON'T RAP LIKE THIS, EVER, PLEASE.
The term's origin was invented in argentinian freestyle competition circuits, with many rappers maintaining this style of rapping. You can see much of this style being exploited in events such as Red Bull: Batallas de los Gallos and FMS (Freestyle Master Series).
Man, this four-bar is wack, he ain't rappin' about nothing, boring af.
Nah, I don't listen to rap that much anymore, everybody's a four-bar nowadays.
When something is extremely unusual and awkward. This term is used when there is no other possible words or phrases to describe the action, person, place, thing, or situation. It is usually saved for the most odd and weird incidents or people. It's origin comes from the very unusual act of soberly shoving four fingers into a drunk, fat girl's anus.
Homie - "Dude, he totally just shoved his whole hand up her asshole."
The Four-Way Barber Shop involves a girl taking it from behind, in front, and giving two hand jobs. Upon completion, all four guys (bonus points if synchronized) cum in her hair and style into an updo.