I've been watching a lets play by a known fallout LPer.
It seems that...
1 - Things are more focused on combat, and less focused on storyline based "world building"
2 - Things seem a bit more "rail-roadey" in such that Fallout 3 was this way, as opposed to New Vegas, which was far more "open"
3 - The way Power Armour is introduced and used, and the way crafting works, indicates a significant shift towards combat and away from RPG elements.
IMO all of these are bad things.
1 - The game is radically different when it comes to physical "world building" and is beyond compare to others, with Skyrim's HearthFires being the only one that could possibly come close
2 - Watching the exploring of the world is extremely entertaining. I may be missing something, due to watching and not playing, but exploration seems top notch, and this is either the best, or tied for the best in Bethesda's games when ranked for exploring the world
3 - Radiation has changed from counting rads, and suddenly giving you X or Y if you pass from 199 to 200, to a system that effectively "caps" your "max HP" for a time.
IMO all of these are great things.
Based on what I've seen so far; if New Vegas is a 10, Fallout 3 is a 6 and Fallout 4 game is a 9.
Compare to Skyrim which would be a 9.75, and Oblivion which would be a 7.5
IMO of course.