Empire managed to sit down and talk with Matt Damon about (among other things) a fourth Bourne movie.
On whether he'd be returning to the franchise:
I read online that they're doing another Jason Bourne movie with Tony Gilroy directing that I'm not in. No-one told me, literally nobody called me. People think I have insider information, but you get a sense of where I am in the pecking order!
On what would make him do a fourth:
I'd do it with Paul [Greengrass]. Universal don't actually own the Bourne character - the estate [of Robert Ludlum] does - so technically I could go to Warner Bros...and Universal could read about it online.
The Bourne Legacy is sill expected to hit theaters 17th August 2012... although exactly how that will play out without Damon's involvement is unclear.