Well, as much as I'd love to have more Avatar, in the end, I really can't see a 4th season either. As to the original post on this long thread of debatingness, the titles of he seasons really are up for grabs, they could represent both the elements Aang masters and the location where the action takes place. It's really a moot point anyways. Either way, Aang isn't gonna have to "master" airbending and it's not like they could write a season focusing around the air tribes anyways. Personally, I feel that the geographical area interpretation of the titles is most reasonable. Book one focused around the water tribes, and ended with the Seige of the North. Book two focused on the Earth Kingdom, and ended with the fall of it. Book 3 has been entirely in the firenation, so far. But, it's also true that Aang mastered waterbending in book one, earthbending in book 2 and firebending in book 3 (we assume).
In reality a season 4 is wishful thinking. Personally, though, I could see an extension on the season 3. What's left, 12 episodes to conclude the series? It seems like that's gonna be hard, especially if tehy intend to contain any of the usual fillerish episodes that are so common in avatar. There's alot of stuff that really needs to popup again... The White Lotus thing, bloodbending, still waiting on an advanced airbending technique, though that'll probably be used against the lord.