Anyone with eyes and a brain can clearly understand that FO4 is nothing like FO3.
As for the engine, I wish people would get this straight: BGS' engine (Creation Engine, formerly Gamebryo and prior to that NetImmerse) is the ONLY engine on the market that can do what they do (i.e., have hundreds to thousands of interactive objects throughout an open game world). If you can make a better engine to do this, please do and I am sure that Bethesda would be very happy to consider buying the source from you. Otherwise, understand that there is NO other engine that can do their games. It's that simple. They've looked, believe me. If they used another engine, you'd have 1) a far less interactive and living world, and 2) a game that performed far worse than what they have offered throughout the years.
People need to understand that BGS knows what they are doing and certainly has the money now to use any engine they can find that would do what they need to do. There isn't one, that's all, except for theirs of course.
We need to ignore Undecaffinated as he is obviously a troll. BGS deliberately refuses to simply make the same game as sequels, unlike many other developers, and the outrage of their consumer base for changing things with each game is more than enough to support this fact. One thing that anyone can count on with any new BGS game is that it will not be the same as their prior games. If you just want the same old game with a few tweaks to graphics and such, don't play BGS games because they are the exact opposite of that.