On one hand, I've not played Fallout 4, so I can't compare the two quite honestly yet. Take what I say with a grain of salt.
That said, Fallout 2 is a bad game in my mind. I liked Fallout 1, even though it had it's flaws. Fallout 2 on the other hand took what made Fallout 1 great (it's story) and completely raped it with a pitchfork. The main quest alone is so full of holes it's ridiculous. Then they didn't bother giving it better gameplay (which means it shares the clunkiness Fallout 1 had), it somehow looked and sounded uglier than the first game (Vault City was alright, but the rest... And people complain about brown and grey shooters these days, what about brown and grey TBRPGs?) and the difficulty curve was more like a difficulty cliff. Games, IMHO, should either start easy and get progressively harder as you encounter stronger and more dangerous enemies and obstacles. Fallout 2 does not follow this premise, believing it to be for silly, whiny babies. It starts off hard as balls and only gets harder with every passing hour. And that's if you're not trying to go for the best endings, which make the game even harder! But why would you want to go for the best ending, when this story is riddled with more holes than swiss cheese and none of the characters in this game do anything to make me care for them?
On the flipside, Fallout 4 has horrible C&C mechanics, a bad story and a terrible dialogue system... but it looks like it's got much better gameplay, it looks amazing, I like the soundtrack, and from what I've heard, the difficulty curve isn't going to make me rip my hair out in frustration! It's not a worthy successor to Fallout 1 or Fallout: New Vegas, but I'm sure as hell going to enjoy it more than the mess that we call Fallout 2.
TLDR: I despise Fallout 2, and Fallout 4 looks like it's going to be better IMHO.