If the forthcoming The Bourne Legacy is good then it'll be one helluva quadrilogy.
With no signs of stopping, either. Like James Bond, there is plenty of source material for continued films in the franchise. As far back as The Bourne Supremacy
film in 2004, there was talk of the film franchise eventually being 5 total films; since then there have been a total of seven continuation books after Ludlum's original trilogy. I haven't read any of the continuation books, but Identity
was the only one loosely based on Ludlum's books anyway. So much like Bond, they're winging it.