The Sims 3 was starting to run annoyingly slowly on my PC (I'm talking half-hour loading times, almost as long to save and horrible framerate issues in most neighbourhoods - looking at you, Bridgeport) so I was super glad when The Sims 4 came out.
On the plus side, it runs beautifully. The sims themselves feel much more customisable - everyone I made in The Sims 3 ended up looking like a pudding face, which I suppose was just the game's style, but I think The Sims 4 feels more realistic in that respect. Multitasking and group conversations are the best things ever. @Lindsaysoderberg, I totally agree about the traits. I was kind of disappointed when I realised there were only three, but the extra traits you can get from aspirations evens that out a bit and they do seem to have much more of an impact on the game.
I'm still not sure how I feel about the lack of toddlers. That life stage was pretty annoying, but it added an extra level of realism and challenge. I'm also disappointed that they took out the open world; I'm sure that not having to track every other sim in the neighbourhood is what's making it run so much better, but I miss being able to completely control two different sims on opposite sides of the town. The option to have them build skills while I'm away is nice, but I feel like it's a bit glitchy? Sometimes I have them work on skills for a whole day while I'm out and it doesn't change much at all, but other times they shoot up ridiculously fast in next to no time. I had one chubby sim build athletic skill while I was gone and a few hours later she was stick-thin.
Since I bought The Sims 4, I've found myself alternating between it and The Sims 3. I think the reason is that The Sims 4 is a better, more advanced, more polished game, but it lacks a lot of the detail that The Sims 3 has with all its DLC and expansion packs. I'm sure that eventually I'll never want to play The Sims 3 again, but for now I'd be hard pressed to pick a favourite.