You are aware Bethesda was contracted to make 3 full Fallout games and DLC?
So far they have only done 2 games, though they didn't make New Vegas themselves, it is still counted as one of theirs.
You are a bit confused and that's irrelevant now anyway. Bethesda originally licensed three Fallout games from Interplay. That simply meant they had the right to make them, not that they were required to. But it doesn't matter since Bethesda later bought the franchise outright from Interplay and licensed the right to make an MMO back to Interplay as part of that deal. The MMO situation is legally in dispute, but either way Bethesda now has full control over other Fallout games.
That all said, I have no doubt we will see Fallout 4 in a few years. Bethesda is likely already working on it since the last stages of finishing Skyrim are more technical in nature. The writers and artists at Bethesda have already moved on to the next project, which is most likely Fallout 4.