While creation of houses and the visual aspects of sims has greatly improved in 4, actually playing sims is not fun at all in sims 4. All sims act the same, they don't do anything interesting on their own.
They have less traits and they have no real impact on their behaviour. Far less trait options too.
I've really missed sims actually taking initiative, making friends and enemies on their own, choosing their own partner. Not taking hours to eat a simple meal. No load screens. Body hair for men. More and better supernaturals. Much more things to do still, despite both games pretty much having an equal amount of expansions by now.
Sadly all sims 3 content is still rediculously overpriced, otherwise I'd get some stuff I didn't have before but it's not worth what they're asking for an old game. There's flaws too but still, sims 4 offered only a graphical update and better pathfinding. Everything else went backwards.
Pretty much all problems that sims 3 has where carried over to sims 4. Like sims having a build in phone but they don't seem to obsess as much over it in sims 3, seems more to do with their traits. Or obsessing over the computer.
But still, main thing I take away from it is that I made a group of eight sims to put through college. I gave them all varied personalities. And they mostly actually act like intended. In sims 4 no matter how you create your sim, they can only be a vapid hipster. Nothing else.