My Sim was pretty thin coming out of CAS, but it seemed she gained weight rather rapidly. I bought her a treadmill and she uses it daily. Now it seems like my Sim has become extremely muscular, but ...
I think one thing with TS4 is their needs turn yellow when they are only halfway down and if you're like me, as soon as you see a need turn yellow you want to turn it green again. So we end up over feeding our sims. I've had to make myself ignore their need for food a bit longer and get on the treadmill a few times a week, that seems to help.