A.M.
I also have a daughter about to turn four. She hasn't used the toilet since baby sister was born so I was convinced it was regression. She never complains that it hurts, we could never get a reason why she was having accidents. I finally took her to the doctor this week, told them I was 85% sure it was intentional but could they check to be sure, and they are running tests.
So, long-winded, I'd have them do a urinalysis because it doesn't have to be a bladder indfection to be a physical problem. HTH.