I play both, although Sims 4 more often these days. I still play Sims 3 because I've got long standing projects there and it has some fantastic qualities that I'll enjoy time and time again. I also still create careers for Sims 3, that helps ;)
Mostly these days, I'm playing Sims 4 for the pleasure of playing. I could never really do this with Sims 3 - there was always a problem, or a limitation I couldn't overcome. The simplest ideas ended up being exhausting because I had to go all around the houses to make the idea work; not worth the hassle, sometimes. Sims 4, however, has the quirkiness I love and the depth of character (I think I'm a minority here (as usual) because I do see the depth in their characters in Sims 4. I never saw that in Sims 3, though). With the Sims 4, I play for hours and with very few mods (none of the big ones, I don't feel I need them). <3