UPDATE - FALLOUT 4 RELEASE DATE REVEALED?
At this point it certainly wouldn't surprise anyone to know that Fallout 4 was in active development.
It's been more than 4 years since Fallout New Vegas arrived and three years since Bethesda released their last major title in 2011's Skyrim. The fanbase for Fallout is still ripe and the world, it seems to me, is more than ready for another detailed RPG with a well-made world and more quests than you could reasonably count.
Whispers have already begun about a major Fallout-related announcement for E3 2015 and now an insider is apparently speaking out. Chatting via email to gamercenteronline.netthey said the new game will absolutely be confirmed at the event this summer and that it could launch as early as Spring 2016. The other major news was that Fallout 4 won't grace older machines - its going to be an Xbox One, PS4 and PC show only.
Of course this is all entirely speculative - with no concrete details and no way to verify the source. It also seems like pretty much a foregone conclusion that Bethesda will announce something major at E3 this year, and its reasonable to expect a Fallout title, given the long wait since the last one. Still, we're definitely interested in seeing if war will ever change...
Some new rumours are suggesting a May 2016 release date for Fallout 4 - more here.