More speculation on a 4th film, but nothing much as of yet
 

Cameron Reveals More On Avatar 4 | Movie News | Empire

Seems likely the original planned story will go up to the third... I just really hope a 4th doesn't do a Star Wars with it. On the other hand, would be nice to have Grace's backstory.

This will definitely be overkill. They should leave it to 3

Grace's back-story would be interesting, but that's not really "Avatar." At least with Star Wars the story kept to something we all wanted to know about, namely who Darth Vader really was. With Grace... it's just not the same. It'd be better if it was about the actual discovery of Pandora and its human populating, along with Grace's back-story.

But if it's just about Grace then I don't think it'll fly too well. It's not epic enough of a scale, whereas discovering Pandora is.

I wouldn't want a 4th film prequel if it was an option. I'd rather have the trilogy and a prequel novel of the first three decades on pandora covering Grace's and Neytiri's exprence, and perhaps the time when Jake lost his legs.

I like the ambition more than anything. Even if the prequel turns out crappy (even with all this planning and foresight), at least it was done. I'd rather have the chance to see a good prequel than hope it isn't made because it might be crap.

i would just leave at 3 if i were him but you know my name isn't James Cameron

I think it would be interesting to see what was happening on those first RDA expeditions to Pandora. Like a little history lesson.
I mean, we all know that the end result is: the problematic relationship between the Na'vi tribes and the RDA. Though it will be exciting to see what the initial contact between human and Na'vi is like.

^This. :)

I (as we all do I guess) have vague ideas about how the sequels will go, but pretty much no idea of what a potential specific storyline would be. I can easily imagine what a prequel would consist of though, since there's significant backstory in Avatar, so I really hope JC will make the story into something more unpredictable.