http://www.boaphileplastics.com/rhin...swsweaters.jpg
This is what people mean when they refer to a "tub". The tub can either be by itself or in a rack system as seen above. For my first BP, I had her in just a plain old sterilite tub. The rack system for Ball Python is widely used because it's much easier to control humidity and temperature than in a tank, and its just convenient, takes up less room, and easier to clean. I'm sure there are many other reasons besides those listed.
Anyway - a dehydrated snake will have wrinkles. As long as humidity and temperature is good and clean water is accessible, she shouldn't be dehydrated.
Currently most of my females are off feed either due to breeding or because of the season. If your girl is over 4lbs, that means she is over 1800g, and that's nice and big. I wouldn't worry about weight unless her spine begins to stick out very prominently. Over a year ago my 2200/2300g female went off feed for 10 months and was fine.
Sometimes I have to use live mice or rats to jumpstart them back into feeding. Sometimes putting them in a smaller enclosure works, sometimes using dirty gerbil bedding to scent their cage before feeding works.