I'm looking forward to that too.
Bethesda has pointed out that Fallout 4 is also Elder Scrolls 5.5 in a lot of ways at least as far as the sandbox world.
I think they are taking an improved Creation Engine upgraded to be a GOOD FPS with a much opener world and fewer load screens and then pulling in the best mods and features of F3 and FONV.
It certainly doesn't feel like they rushed it.
And you have to admit improving and expanding an existing game engine is much easier than creating a brand new one.
They are basically pulling an Obsidian.
By using an existing engine which you know what it can and can't do, they could put most of the team on making the best Fallout game they can.