its code for when your at a party and you need to someone to check if you are armpit sweat is coming through your shirt. Or its used to tell someone that your are armpit sweat is coming through your shirt. Or can be used to explain how hot it is at a club, bar, party, and etc.
Dude 1: Am i number 4'ing?
Dude 2: How hell did you do that through two shirts.
Guy 1: There so many people at this party that its making me Number 4 up in this bitch?
Guy 2: I heard that, Old Spice 8 hr protection my ass.