I am slightly concerned about them just rehashing Fallout 4s engine, as it seemed already old when that game was new..
There is nothing wrong with Fallout 4's engine. It was just poorly used, at first, in FO4.
Since the game was launched there is a HD pack and all sorts of nice stuff offered up by Gamedoesn'tworks to make it look better. And it does, especially driven to 4k. I posted a couple of screenies recently and it really is like playing a different game to the slop that was launched.
Do note these have suffered at the hands of JPG compression. When I first started playing the game that ^ off in the distance would be a blurry mess.
If you are not happy with the graphics then either pump up the resolution by DSR/VSR (if you are on AMD) or by buying a new monitor. Then, get a high end card like a 1080Ti and enjoy.
The problem with a very large open world engine is.. Well, it's kinda like that saying from the movie Lock Stock etc. "You can have a gold plated Rolls Royce, so long as you pay for it". IE, taken in the correct context you really can have Fallout 4 looking as good as you want, providing you have the hardware to run it.
It was exactly the same with FO3, too. When it came out it looked balls, then a 1gb HD pack came out but the problem was you needed 1gb VRAM and a GTX 280 just to run it at acceptable FPS.
BTW guys and girls.. You should automatically assume that any gameplay you are seeing of the game is it running at 4k on max settings on very high end hardware. That will be a given.