this is more of an "initial" if you will. it's basically saying that that person using the "tag" is passed the noob-days and now a pwner (im proud of you dude/chick).
most people confuse this for azn (witch means asian). do yourself a favor and don't look like a noob when you say to someone that he/she is an asian with 4zn at the beginning or end of their name.
YES it's true that 4zn means asian in some games, but the meaning has changed for there only will be one AND ONLY ONE initial for every one (pwner, azn, 4zn, BR, etc).
random people playing Halo (or something else... use your imagination):
random peep: "fuck dude you got me again"
4zNMrPibb: "it's just who i am, noob"
random peep: ".... i hate you"
4zNMrPibb: "yeah... your mom regretted saying that too"
random peep: "aww your mad..."
4znMrPibb: "XD wtf? how noobtacular of you"
Source: Mr Cat In The Hat, i dont even know
A pro sniper that plays C&C Renegade!!!
Random guy: Omg, can I be like you?
4zN: No.. I am too pro!
Random guy: Ok... fine...