Hey, new member making first new post here--
Looking at Fallout 4 it's looking like it's definately going to be situated in Boston, making it in the "capital" I guess, of the Commonwealth (as seen on the fallout wiki's 'Commonwealth' page which encompasses New England).
Fallout 4 undoubtedly would stick to Massachusetts and the immediate area. But with regards to the larger scope in the post-nuclear setting, I'm wondering if the 'Commonwealth' -- with the United States now gone -- would naturally travel from New York in the south and NY/Quebec in the West all the way up into the Canadian Maritimes. I'm from the largest city (Halifax) myself and, frankly, if the current poltical borders got rearranged of both the US and Canada and Boston/Maine severed ties with New York and Washington (which has essentially happened in Fallout through no conscious poltical move but a consequence of the bombs), the Canadian Maritimes, or at least Nova Scotia's mainland would undoubtedly seek to sign on with Boston rather than stay in any sort of Canada.
There's ties to Boston in both history and heritage despite being on opposite sides during the American Revolution and even after the establishment of the US and Canada. I'm not sure what particular history the annexation of Canada entails, but frankly if Boston approached it independantly, a lot of the Maritimes probably would have welcomed them and after a post-nuclear incident I could easily see identifying with the Commonwealth or wanting to gain admittance.