The only thing I see having real appeal is the create-a-sim process. I really like the customizability they've added... but gameplay doesn't seem to have improved at all - aside from multitasking i really like multitasking.
And they have emotions now. They'll decide whether or not they want to do things, somewhat randomly. It was bad enough in Sims 2 when the teenagers wouldn't do homework because they didn't want to.
I like Sims 2 best I think. Sims 4 would have to get down to crazy low prices before I'd think about getting it.