They can go over a year without eating. 4 weeks is absolutely nothing to them.

No need to worry or change size.

What you do need to do is make sure that your BP has proper temperatures/humidity/hides these are the building blocks to having a happy BP.

Also make sure that you heat the rodent up to 100 degrees. No more, no less. 100 degrees is the body temperature of a rat so it is the temperature that they look for. Too cold and your BP won't even notice it, too hot and you risk cooking the rodent... And that leans to numerous problems.