Hey , I have what might be the possible location of fo4 or atleast what beth is considering.
http://www.appy-geek.com/Web/ArticleMobile.aspx?regionid=4&articleid=4004128

I have no idea the credibility but I thought I should share.

And yes I know I spelled possible wrong in the title

Boston, huh? I can't wait to see how they manage to bring BoS, Enclave and super mutants there

There's a thread for speculation here: http://www.gamesas.com/topic/1404746-fallout-4-speculation-suggestions-and-ideas-thread-82/