I play Fallout 4 on my 60 inch 4k tv and I play the game at 4k. My situation is different than most because my screen is so big(not meant to be a brag by no means) 1080p looks worst than on a smaller tv due to pixels per inch.
In any case, Fallout 4 to me looks like a step up from skyrim graphically in most cases.
The Theory: I think the non "graphical leap" might be for a reason that most people didnt quite think of...
Elder Scrolls is Bethesda's baby. And while Fallout 4 is part of their circle now but their original series is Elder Scrolls.
So what if Bethesda looked at what was to come and put it all on the table to make the decision to not focus too much on graphics.
What if the reason Fallout 4 doesnt look up to par with "next gen" because it would bring down the "Wow" factor of their baby Elder Scrolls. What if the "graphical leap" was planned to happen once they got acquainted with the new tech?
http://www.playstationlifestyle.net/2015/11/03/fallout-4-development-started-elder-scrolls-v-skyrim-ported-xbox-one/
With the next Elder Scrolls Bethesda should have Directx 12 available to them. Its up to them to modify or create an engine that fully facilitates the new technology.
What do you think?