I love FNOV, I've played over 1200 hrs with 6 different characters over the last few years. That said, I've played it heavily modded from the Nexus, which dramatically improved the base game with not only graphical updates, but AI improvements, NPC improvements, new NPCs, better crafting, ect. So a modded FNOV is at this point a better game (to me ) than what FO4 is currently. But that's an unfair comparison. I think a year from now, once the creation kit has been out and the mods start flowing, FO4 will be the equal or better than FONV.
Comparing un-modded base games of each, I give the nod to FO4. I haven't finished it yet, and I'm up over 200hrs, but here are some of the things I like the best.
1- Settlement building. I love it, but it's got tons of little quirky bugs that have been well documented on these forums. I know Bethesda will fix some of them, and modders will greatly improve the whole building/supplying/defending scenarios.
2- Atmosphere, especially exploring cities/towns/factories ect. is awesome, often tense. FNOV was more out in the open exploring.
3- NPC Companions. This is kind of a toss up between both games when comparing the base game. Mods greatly improved the Companion of FNOV, and I'm sure that will eventually be the case with FO4. Right now the interaction is just a little to sparse and needs to be fleshed out more.
4- Combat is just awesome. It requires planning in many cases of you'll get your ass kicked quick. It can also be fast and furious. Awesome guns.
5- Crafting, comparing base games, it superior in FO4. I love the system, although it also needs some work to make it better. (Modded FNOV crafting at this point is superior)
6- hardcoe mode. An option that FNOV had at the beginning of the game that required you to eat/sleep/drink within a certain time period or your stats would decrease. I loved that, it provided an environmental challenge to the game. The nod goes to FNOV on this one for having it from the start.
7- Factions. I'm uncertain at this point, though they are crucial to both games. I haven't gone through enough in FO4 to offer a true opinion.
These are just thoughts off the top of my head, I'm sure there are more comparisons to be made.