N4L is shorthand for the phrase "Nerd (or Nerds) For Life". It is used among nerds as a confident self-reference or as a response to an insult to one's nerdiness.
Jock: Psh you are such a loser, Star Wars is old, lame and stupid. Why would you wear a Greedo shirt to school?
Nerd: Say what you want, I'm a N4L and embrace that fact!
Texting Nerd 1: Hey you wanna watch some BSG tonight?
Texting Nerd 2: Of Course!!! N4L!! :D
NFL meaning nigga for life.. used mainly amongst black teens jokingly. like BFF
person 1: Hey, person 2, you know that girl over there?
person 2: Yeah das like my N4L Crystal.