Is it in developement? Maybe. However Bethesda hasn't said anything about future Fallout games, or in fact future games of any sort. As for rumors, there are a few educated guesses that can be made but nothing substantial, and there's a lot of stuff floating around which is made up, pure and simple. It doesn't help that there's at least one fan video floating around pretending to be a FO4 trailer.
We have http://www.gamesas.com/topic/1395245-fallout-4-speculation-suggestions-and-ideas-thread-74/ for FO4 speculation (and suggestions). You're welcome to browse through it and join the discussion. We also have http://www.gamesas.com/topic/1385211-official-future-fallout-locations-suggestions-topic-16/.