One who, through browsing 4chan or other related and similar material and have discovered the joys of anonymity, has picked up the unfortunate habit of calling every one else a "fag" of some description. This habit often lures fellow 4-fags into labelling the original 4-fag as a "fag" too, which inevitably leads to one big loop of slandering each other and every one else. Would probably be looked upon as a serious insult by most 4chan followers, due to the similarity in sounds to "fur-fag".
A close variant is 4-tards, here substituting "tard" or "retard" in place of "fag". Others would also claim a third category of "Anoni-fags", which would be those who would use the Anonymous name just to insult some one in much the same way as a 4-fag would.
As a matter of fact, this probably counts as 4-faggotry, just by posting this. Damn...
God, I wish that 4-fag would stop posting those shitty comments on my youtube videos.