I think they might be running into delays because they have probably realised that they have run 2 Elder Scrolls games and a Fallout game (plus Obsidian's excellent Fallout New Vegas) on roughly the same engine.
So they are now having to start from scratch and building a wholly new engine for Fallout 4, Elder Scrolls 6 etc. It's like Squenix with Final Fantasy XV, they are having to not only develop a new game, but also have maybe had to scrap and reinvent the engine running it.
So I predict Fallout 4 will happen, and having seen the news about E3 I expect it will be out in 2016 with Elder Scrolls 6 coming a year later.