I play both. I can play Sims 4 for a few hours and have fun; I can play Sims 3, look up after awhile, and realize that 8 hours have passed and I think need to turn off the game and try to get at least a couple of hours sleep.
But with Sims 3, it was not designed for modern computers and for me, I have to do a few things to get it to run properly. I use an FPS limiter because Sims 3 does not come with one. I run both Sims 3 and 3booter (the FPS limiter I use) in Windows 7 compatibility mode -- if I don't, Create a Style lags. I use the NRAAS suite of mods, and I also use Awesome Mod, but I don't recommend Awesome for newbies to modding because the people at that website are, well, hostile and proud of it. The people at NRAAS are great, though, and incredibly helpful.
I always am confused by people saying Sims 3 sims have no emotions -- they're not in your face, and they are governed by moodlets, but they're far more subtle. For instance, if a Sims 4 sim catches her spouse cheating, she may get jealous, she may get angry, or perhaps the house will make her happy. In Sims 3, a nice atmosphere will lead to increased mood, but if a Sim 3 sim gets cheated on and knows it, no matter how high the mood is, the Sims will be mad/hurt/whatever at their partner. It's possible they'll breakup without the cheater doing a lot of work to try to make up. You definitely will not have a betrayed sim smiling and chatting or getting over it in a few hours.