Actor Matt Damon is working on a fourth installment of the successful Bourne movie franchise. Damon and director Paul Greengrass will be developing an original script for the popular series' fourth outing -- as the films have dramatized all of Robert Ludlum's best-selling novels. Yet both are currently held up on other projects -- and Damon remains cautious about reprising his physically intense role as super-spy Jason Bourne. He tells Parade magazine, 'We're working on it. This time it would be from an original script rather than a book by Robert Ludlum, but the director Paul Greengrass is busy and I am too, so we'll see what happens. 'When I did the first movie I was 29 years old. I am 37 now. After a tough fighting scene, the next day you wake up and feel your body more. That's just the way it is.'