Matt Damon (Jason Bourne) and Paul Greengrass ("Supremacy" and "Ultimatum" director) have not yet signed on to do a fourth Bourne film, but one is still currently in the works.
Frank Marshall, producer of the films and also owns all the rights to the Robert Ludlum library, wants to make a fourth film, but not based on any more Bourne books.
See, Ludlum only wrote the original trilogy, but many other authors have been allowed to write from Ludlum's original works and to continue the adventures of Jason Bourne.
According to UGO Movie Blog, "Frank Marshall does not have the rights, per se, to Bourne. He has the rights to the works of Robert Ludlum. There are plenty o’ Bourne books out there that have yet to be made into films - but they are not Ludlum and, hence, not gonna be coming out from Marshall and (one would suspect) Damon/Greengrass. "The Parsifal Mosaic" is not a Bourne book - but it is a Ludlum spy novel."
Marshall wishes to make the foruth Bourne movie based on "The Parsifal Mosaic."
In an interview done by ComingSoon.net with Marshall, the producer described "The Parsifal Mosaic" as a story that "deals with a U.S. spy who falls in love with another spy who is revealed to be a double agent just before her death. When he retires from the business, though, the spy learns that she may still be alive and sets out to learn the truth."
Other interesting parts from the interview.
Q: Would this come at all from the new book, "The Bourne Betrayal"?
Marshall: No, the "Bourne Betrayal" was written by a friend of Ludlum's.... It had nothing to do with us.... We want to stay true to the original character.
Q: Any ideas for a "Bourne" title?
Marshall: Well, there's a lot of jokes. "Bourne Again," "Bourne Yesterday," "Bourne Free," "Bourne on the Fourth." I actually want to do that. Call it that and release the movie on the fourth.
More news to come! Catch ya' later!