Does it seem likely that a professional voice actor would break his NDA?
Considering how successful the Modern Warfare series has been for Activision and the Call of Duty franchise, it doesn't require a mental reach to realize that Modern Warfare 3 would surely have a sequel, and another… and another… until the series no longer proves financially successful. Really, the only suspense left is as to when the next in the series will be released, and even that's hardly anything to blink at. After all, the Call of Duty franchise releases are like clockwork, returning every holiday season.
Unsurprisingly, according to Captain Price voice actor Bill Murray, Infinity Ward's next project is Modern Warfare 4. Murray allegedly told this to ThisIsXbox founder Jason Andrews aboard a plane, when Andrews recognized Murray by his voice. Andrew claims that Murray, not knowing Andrews's media affiliation, said the following: "Yeah, on Monday I am off to meet Infinity Ward about the next game, Modern Warfare 4, I’m doing work on the sequel to Modern Warfare 3, it carries straight on and I only ever appear in the Modern Warfare games…"
The validity of whether or not Murray would say such is unconfirmed, although Andrews posted a picture of Murray and himself onboard the plane as proof. Still, it seems rather unlikely that a voice actor would break a NDA just to impress someone he thinks is a fan.
Soon after, Infinity Ward tweeted that what Murray stated about working on Modern Warfare 4 is false. The developer stated that it hasn't yet spoken to any voice actors, so "all news is speculation."
Notably, Infinity Ward didn't deny Modern Warfare 4's existence. As expected, we'll probably be seeing Modern Warfare 4, whether or not Bill Murray's voice is featured or not (unlikely, considering Captain Price's integral role), come Holiday 2013.