My personal views:
Sims seem more alive in Sims 4. They are much more expressive.
The food looks more delicious.
Everything is prettier and easier on my eyes - I, for one, love the cartoonish feel.
Toddlers have much more life to them. I feel they are rather boring in Sims 3.
Pets are better looking and you can do lots more stuff with them. You can be vet and have a clinic!
Gardening plants look a lot better, too.
Water looks and sounds more appealing.
I enjoy clubs very much as well.
Few routing issues. In fact, my computer runs Sims 4 a whole lot smoother, than it does Sims 3 with all its optimization and routing issues.
Sims can be cheating in the same house without anyone noticing, unless they are in the same room.
The bar and drink/drinking visuals are an improvement for sure.
Romance interactions are so much better in Sims 4.
Grim Reaper looks and sounds better.
Mermaids are prettier.
Vampires are cooler with two forms, customization, and have better and more immersive interactions.
I could go on...
So, all in all, there are a bunch of gameplay and visual features, as well as technical ones, that I find easily outdo the Sims 3 :)
