Yeah...you do realize all of those articles aren't exactly recent, save for the last link (and even then it's always best to take news you find on a non-official forum with a grain of salt anyways). Besides, a live-action film of Avatar with Shyamalan is a very creepy idea, especially if it gets made... There's plenty more of Avatar to explore to easily warrent a 4th season, and just because Nickelodeon hasn't ordered a season 4 yet doesn't mean there won't be one
Okay, so the articles aren't recent. But at least I'm coming up with links, right? The only thing you have going for you is that Air comes after Fire in the Avatar cycle, and that's not exactly persuasive.
Also, why is everyone convinced the season will end with Book 3? Anyone ever entertain the possibility the Comet might not even arrive this season?
Here's what Avatar Roku said in the eighth episode of the show: "Sozen's comet will return
at the end of the summer, and Fire Lord Ozai will use its power to win the war once and for all. Aang, you must defeat the Fire Lord before the comet arrives!" The comet will arrive this season (Book Three: Fire = summer).
You have to admit, the show is usually ambiguous about what day/week exactly it is, unless specifically mentioned (ie, next episode takes place 4 days before the Eclipse). And ok, even if the comet does come at the end of Season 3, what if Aang fails to stop it? There's easily enough from that idea there to go on to Season 4.
The comet only comes around once every 100 years. This is Aang's only chance to stop it - if he doesn't, the world is basically done for. And since he's the hero of this show, he WILL stop it. Besides, Roku told him that if he couldn't stop Ozai in time, even the Avatar wouldn't be able to restore balance to the world. You should watch "The Winter Solstice: Part II" again, that's where these facts are coming from.
So yes, 4 seasons in my mind still makes perfect sense. 4 elements, 4 seasons. I mean, why else bother naming each season after an element in the Avatar cycle? Besides, Avatar is easily the best show Nickelodeon has had in a very, very long while, and I doubt they're going to end it after only 3 seasons.
Aang was a full-blown airbender (good pun!) right off the bat. The first season was entitled "Book One: Water" because it was the winter season, and also because that was the season in which Aang learned to waterbend. If we get a Book Four: Air, what will Aang do? No war to stop, no element to master... You suggested that the comet might not come until a fourth season, but I've already debunked that.
If more Avatar seasons are ordered, they will most likely feature all new characters and an all new plotline. It will be a whole different arc altogether. I can't remember where I read that, exactly - I think it was on
Distant Horizons, an Avatar nerd forum that I frequent.