I think it compares because they have a similar focus...the Sims themselves. For me TS3 was completely devoid of personality, it was just a big expanse of stuff rather than enabling you to connect with your Sims. TS4 seems to have gone back to 'basics' and focused in again on the Sims, making them more quirky and likeable.
At this stage they seem similar, small world, likeable Sims, it will be interesting to see how TS4 progresses. I personally like the new approach with TS4!