Didn't play FO4, but I really did not like FO3 much.
The thing with FO3 is that the wheels start to come off after Megaton.
The writing is simply atrocious, and the main quest design is bad and linear.
I think another problem with it is that its such an easy game for mechanical reasons. Like how Bethesda uses DR instead of DT, so even someone in heaviest armor can be plinked away with any shitty gun. Which means, your shooting skill is your greatest asset. The only way Fallout 3 manages to create difficulty later on, is throwing bullshit enemies with ridiculous magical resistances and damages, and enemies with ridiculous HP bloat.
It has lots of exploration, tho, but honestly I think the exploration in FO3 is too... autistic. Like, the game is too easy, why would I care about going around exploring? To get loot I don't need and then obtain caps I don't require to buy things I already have?
Meanwhile in FNV, exploring brings you nice goodies AND makes your character stronger for the coming challenges, because the game has less pity on you than FO3.