I thought the TS3 was a huge upgrade personally. Even after years of playing TS2 then an intermittent period of no sims games. I did not find TS3 hard to pick up at all. If you go in expecting a revamped Sims 3, you're going to have a bad time. If you go into the Sims 4 thinking about it as a new game with a new play style that you are excited to learn, you'll have a great time with it.
Some people still play the sims 2 and don't really like the third one that much. If you end up playing and it's simply not your cup of tea, that's fine. It's a new game, and I know it can be intimidating to learn a game you've already mastered all over again, but people made the sacrifice for the third sims. How do you place lots? What do you mean you don't place lots? Where is the hot tub? What? No hot tub? How do I move the camera around? What's my Sim doing? This game is stupid. However from the Sims 2 to Sims 3, it was hell.
Better graphics, smoother gameplay, some more options for your sim.
I remember the switch from The Sims to The Sims 2 was smooth, because it was basically just an upgraded The Sims. My mom and I have been avid Sims players since the first one.