Credit: Ben Watts/GQ

Matt Damon would have pocketed an estimated $20 million to appear in a fourth Jason Bourne film, but in a shocking twist, he walked away from the $1 billion franchise. Why?

"If you look at the first three movies, we kind of pounded that idea of identity and amnesia into the ground. We really got everything out of it that we could," Damon, 41, explains in the January issue of GQ. "So to reboot it, we need to come up with something completely new."

That's exactly what director Tony Gilroy did; Oscar nominee Jeremy Renner, 40, will appear in The Bourne Legacy (slated for release in 2012). "I'm really pulling for this one, even though I don't have anything to do with it," Damon says. "Selfishly, it's bad for me if that movie doesn't do well."

Damon -- a father of four who currently stars in We Bought a Zoo with Scarlett Johansson -- adds that he still feels "inoculated" by the Bourne franchise.

"It feels like I can swing freely, like a baseball player -- just be relaxed and really do the things that I want to do and not worry, because I know there's another one out there."