I personally feel, after playing through fallout 3 for the first time in a couple years recently, it may take place in the commonwealth. So Massachusetts and surrounding area as done with fallout 3. New York, as well as a few other areas could be covered in add-ons as done with Fallout 3. This is only if they follow a similar formula for Fallout 4. They may want to go back out west though, so my bet would be more north west, maybe Washington state.