In Brawl, past a certain theshold of knockback you could act out of hitstun with pretty much anything except specials and jumps. In Smash 4 you can still act out of hitstun, but it happens much later than in Brawl. When you bounce off of something you can attack/air dodge out of hitstun a lot sooner than usual, but outside of that you only get out of hitstun a couple frames early at best.
The biggest reason you can't combo like you could in Melee is because the characters in Melee moved way faster, had more gravity, and aerials had half the usual landing lag. When you jumped your ground momentum transferred to your air momentum so you pretty much could casually fly into them on demand. The extra gravity made all pillar-esque combos and Dthrow chain grabs way stronger. All aerials having half the landing lag, more lenient AC windows, and could be edge canceled, making them all way better at starting combos.
SHFF aerials due to all the above factors had incredible offensive potential and could easily transition into crazy and stylish death combos. SHFF aerials executed in like half the time, went about twice as far, and had around half the total lag (less air time and 1/2 landing lag). The mobility specs and lag on attacks are just on completely different levels between games. If Melee had Brawl hitstun mechanics and air dodges, its combo potential would still be leagues ahead of Brawl's with Melee hitstun mechanics (aka Smash 4).
All that said, I do think smash is better off without SSB64 and Melee level combos. I was never a fan of extended combos in fighting games, but that's just my opinion. I like the greater emphasis on the neutral game and footsies that Smash 4 and Brawl have.