I don't get it, apart from the changes to the conversation system, I don't see any differences in depth between FO3/NV and FO4, you could argue that the weapon crafting system adds more depth to the formula than either.
Maybe I am just a "shallow gamer"
I feel like people have some crazy rose-tinted glasses for 3 and NV. While New Vegas very clearly had an immense level of variance that makes it stand out, outside of that (and I understand that's still a big conceit to leave out), 3 and NV are very close to 4.
The biggest complaint I see is the dialogue system, which, yes it's got the shitty mass effect "simplification" which is horrible. But other than that, the dialogue doesn't really impact any narrative "depth" any more than Bethesda already did with Fallout 3. It's pretty much the same level of stilted cycling dialogue you got from 3 and less so from NV, it's just in a different coat of paint.
F3 had sufficient depth for me to enjoy it. NV blew it out of the water, all right, but back in 2008, F3 was good-ish in that department and I remember it fondly. I love power gaming and it took me quite some time to skill my character perfectly and get the maximum of skill points out of her.
Well 4 has no level cap and I would imagine anyone (and everyone) will have the entire perk list filled out but 1) that will take a long time 2) you will have finished all the content and then some . This game doesn't have quite the min-maxingness of NV (I'm hesitant to also say 3 but I haven't played 3 in forever and don't remember the skill system all that much). But, like the rest of Bethesda's post-Oblivion RPGs, there's not a whole lot of really detrimental downsides, which is my main issue with the character leveling side of things. Having low int doesn't severely impact anything except for stalling your weapon upgrading progress. Having low str just means you have to juggle your inventory even more than you would already do. I don't feel severely hindered as far as who I can talk with, interact with, and things I can do. Everything is "side ways" progression in that regard. Like, with hacking and pick-locking, or charming/threatening it's just a different alternative to reach the same goal rather than an outcome that could affect the world state.
Speaking of which, while there aren't a whole lot of things that affect the world state of the game early on (unlike Fallout 3's megaton), they're definitely still there.
__________
I'm mostly rambling and and any arguments I have will probably be really incoherent and unclear. It's super late (early?) and I have a headache so I probably can't quite say what I am trying to actually get across.
But last thing I'll mention is...
I really hate this trend of games that are just throwing in dating simulators into their RPGs and then letting people think it's "meaningful character interaction". That's all this and Bioware's RPGs feel like now when it comes to interacting with the side characters. Look at Shadowrun: Dragonfall for how to do side characters perfectly.