Jim was working on the pre-production script and such when we were finishing up the shooting for the Deepsea Challenger documentary at his ranch north of LA a few years back.
We knew at that early date that we were going to be looking at least two more, perhaps three.
Seems like things have gone well, well enough to warrant a 4th movie.
http://www.cnet.com/news/james-cameron-confirms-four-avatar-sequels/#ftag=CAD590a51e
According to Variety reports, Cameron originally planned two sequels, then upped the number to three, then four, after meeting with “some of the top artists and designers in the world” to discuss his plans. Cameron says those who have seen the initial concepts for the Avatar sequels have been left “speechless.”
There is really not much more to the article than this block quote…. So if you are in a hurry, don’t worry, you are not missing anything else.
We will just have to wait and see.