I wouldn't be quite so sure. Notice the mask the guy on the box art is wearing? It has a skull painted on it? Well, so did Simon "Ghost" Riley from MW1 and 2. He died in MW2, but there's nothing stopping Activision from making a prequel exploring his story.
Also, remember this little article from last year? Captain Price Voice Actor Allegedly Outs Modern Warfare 4.
I think there's a high possibility this is set in the same world as the Modern Warfare franchise, but the story might just not tie in directly with those 3 games.





