Another thought - how old and how big is she? A lot of people report that their snakes go on fasts around the 1000-gram mark. And 4 weeks isn't really a long time (even if you're annoyed at wasting four rats).
I would not be surprised if at least some of the fasts people see in snakes that are just approaching adulthood happen because while the snake is young and growing quickly, it has a big appetite and gets fed every week. As it grows up, its growth rate slows and it really doesn't need to eat as often. But keepers still keep offering every week, like clockwork. So at a certain point, the appetite drops basically because the snake just doesn't need as frequent meals as when it was a baby.

Obviously I don't know for sure that that's the reason, but given how often very similar posts come up, I think it's at least plausible.

Also, if she's been in shed for a lot of the time she's been in the new enclosure, she actually hasn't as much time to get used to it as you think - since while in shed, she's probably been mostly just holed up and not coming out to look around. If as you say there are plenty of hiding places and clutter and stuff, and temperatures and humidity are good, you could just leave her alone for awhile longer and offer her another rat in a couple of weeks. Maybe even put her old hide(s) into the new cage in addition to everything else, but otherwise let her be for awhile. If things are really wrong, it's true - she won't just get used to it. But if you keep changing things around constantly, she won't get used to that either.

I'm making assumptions about her size/age based on the fact that you felt the need to upgrade her enclosure, so correct me if I'm wrong. But if she's healthy and has some size on her, a month or two is really not a long time for a ball python to fast. Just wait longer before offering again, so that you don't waste as many rats.