Why would you think one of their biggest cash cows isnt on their minds?
BGS went through great lengths to secure the rights to the Fallout franchise. I highly doubt that they would have gone through all that legal trouble to simply forget about it.
I would be willing to bet that they are farther along in dev of Fallout 4 than you may think.
Think about it, they made a statement that they are now no longer working on Skyrim because a project that they were in pre-development stages is now their main focus. They said they needed the whole team because they are ready to start on full development to create their biggest and best blah blah blah.
This could mean a new title but remember, BGS not Bethesda Softworks was the one who said this. The ONLY games(IIRC) that BGS has made/ developed since 2006ish is Elder Scrolls games and Fallout.
My money is on Fallout.