It is hard to compare Fallout 2 with 4 because the first one is RPG game with survival elements and the other game is FPS exploration game with RPG elements (vanilla game that is).
I have transformed Fallout 4 into survival game with RPG elements tho (using many immersion mods and mods made by myself too, much more satisfying to play than a looter shooter vanilla). 80% less loot, meds and guns, lore friendly PA stats, NPC use ammo, absolute minimal settlement building... and that's how a postapocaliptic FP(S) game should be played.
Okay, back on track, Fallout 2 is a great RPG with a great story and sidequests, but has waaaaay too much guns in it. That always bugged me. And I couldn't do squat about it because I couldn't mod it (not enough skill for that hex editor or w/e). Fallout 4 is as linear as the trans-australian truck road, but is highly modable and can so be transformed into the game you want to play.
Fallout 2 compared to predecessor kept all the goodies and expanded the universe, while Fallout 4 compared to 3 ditched almost all skill and stat checks (bad), reworked the perk system (that's good and bad in my book), ditched weapon and armor conditioning bad), gave almost everyone in a game guns with unlimited ammo (bad), introduced good system for modding weapons and armor (good), reworked PA so it isn't a pathetic suit anymore and introduced power supply for it (very good), replaced critical damage system (good and bad).
I love both games, each one in a different unique way (I have replayed both of them more than five times).