We've known for a while that Cameron was working on two more AVATAR films, but it wasn't until recently that the specter of a fourth film was pointedly evoked (details HERE).
That notion now seems to be picking up further steam, per Showbiz 411's interview with Sigourney Weaver, in which she confirms that AVATAR 2,3, and 4 will shoot at the same time and that she'll be in them. She doesn't know how long the filming process will take, though, indicating: “I just show up"
No word yet on exactly HOW Weaver's dead character from the first film will rejoin the AVATARverse, although one suspects the gag may have something to do with folks' essences being downloaded into the planetary consciousness/global hard drive (apologies for my clumsy characterization) touched on in the first movie. Stephen Lang, who played Colonel Cabbage in AVATAR, has very, very obliquely hinted in interviews that his return to the franchise was a possibility as well. Given what we've seen of his character, I'm not so sure how he'll re-materialize in the narrative, if indeed he's actually involved.
The whole of Showbiz 411's discussion with Weaver can be found HERE.
(yes, I know it's Quaritch...)