Here is my opinion
I think sims 3 and 4 are two very different games. I like both for different reasons and both have their faults.
Sims 3 was a blast at its time and has a lot of fun gameplay. Sims 4 is a blast for me now and I also think it has a lot of fun gameplay and a ton to do.
Sims 3 had horrible performance issues that are not present in the sims 4. Sims 4 is lacking in certain areas and sometimes feels more rushed than sims 3 ever did. You can see the cut corners in gameplay even still.
However as much as I loved sims 3 at the time I struggle to go back to it. Sims 3 has things I wish were in sims 4 sure, but sims 4 also has new additions that make me wish I had them in sims 3. The things I wish were in sims 3 and the general ease of creating in sims 4 just make it very hard to go back and really enjoy myself. Same thing happened with sims 2 vs sims 3 though. I tend to play the current version and rarely return to the older ones.
When it comes down to it though neither game is perfect. They both have flaws but are both enjoyable games even with them. I don’t necessarily prefer one over the other either. I just at this moment in time play Sims 4 solely.