The mollies will be baby machines from now to eternity. It's good that you have 4 females to 1 male, though, because he's very persistent - his persistence will be shared between 4 females, so they'll all get a rest from it, whilst he's off being persistent at another one.
When you say one minute fat, the next not, we should be talking about a cycle of around a month - 6 weeks. A female can go from being fat one day to being slim the next, when she's dropped a batch of fry, but not the other way around.
The chances are that you aren't seeing fry because they're being eaten. If you'd like fry to survive, then provide lots of plant cover in the tank, but please also consider what you are going to do with the fry - cos they won't stay small for long, and with 4 females each having 30+ fry every 4-6 weeks, you will soon be VERY overstocked.