It can hardly be said to be surprising that a Call of Duty: Modern Warfare 4 is in development, but we're unlikely to hear anything official about it until next year. However, in an interview with This Is Xbox actor Bill Murray (not the famous one), who voices Captain Price, 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..."