My kids have not had chicken pox so I don't have any first hand info but when this subject has been posted in the past it seems like a lot of people said that their kids with CAH had no unusual problems with chicken pox.  The only tricky thing is that you want to be careful increasing the cortef if she gets a fever.  If you increase the cortef too much it might affect her immune system which could make her more likely to have complications.  You may want to contact her endo (if you haven't already) for specific instructions.

Another thing you could ask your endo or pediatrician about is Acyclovir.  If given within 24 hours of the onset of rash it may reduce the duration of fever and the severity of other symptoms.   It is recommended for older kids (>12 years) who get chicken pox and children who take large doses of steriods.  I don't know if it would be that much use for a kid with CAH but you could see what your doctor thinks. 

Good luck,  I hope your 4 year old is not too miserable!  Let us know how things go.