James Cameron gave some new pieces of information in a recent interview about the fourth Avatar movie. In the article linked below, there is a video of the interview. These are the main points:
- It will be a prequel.
- It will most likely deal with the colonization of Pandora, the more than three decades before the events in Avatar.
- It will not be shot back to back with the other two movies.
- Cameron currenty only has an idea, but no written script, yet.
He also mentioned that he is still at writing Avatar 2 and 3 and that they refined their production process to reduce complexity during the past two years to have more room for creativity.
Source: http://www.mtv.com/news/articles/1693478/james-cameron-avatar-4.jhtml (includes video)