Regarding the game world, FO 4 clearly beats both predecessors, don't think there's 2 opinions on that (and if there are, i really question the nay-sayers judgement...). Gameplay wise, i guess it's a matter of taste, they left some stuff out, while adding a lot of other stuff, which is of course in the eye of the beholder, whether you'd call the different things an improvement or not (IMO, it mostly is). BUT, the characters in FO 4 are also a clear improvement, compared to part 3. I liked New Vegas much too though. Dunno. Guess you'd have to test it, i'm sure it won't be long until you can get it for cheap anyway. What i'd criticize about part 4 are the very generic faction quests. Really, that's something that doesn't have to be. After the 3rd exactly same mission, you'll be bored, the latest. Also, some of the crafting stuff is pretty pointless, and also too bloated, for example, the cooking. You'll find better items and drugs in terms of effects anyway, than any of the stuff you cook will give you. Also, i'm not sure if building in some "build and conquer" style strategy stuff like the settlements is a good idea. Frankly, i was quite annoyed by it in the beginning, but, got used to it later in the game. Still not sure i like it though. But, if you don't join the Minutemen, i guess you'll hardly care less anyway.
So, tl;dr, Fallout 4 is a clear improvement of part 3 for me, in most regards. Although part 3 is still a good game in its own rights. I prefer the story in part 3, but i definitely enjoyed the game world in part 4 more, and also most of the gameplay. New Vegas is pretty awesome too, but, i think i'd still prefer part 4, simply because it was such a fun for me to explore the game world of part 4.