Hmm, does she eat anything unusual, cos I think I remember reading that if a rabbit eats alot of something like carrots or beetroot it can stain their pee, which would be nothing to worry about.
I think you should probably take her to the vets anyway, just to be sure. Are you going to breed from her, cos if not then you might as well get her neutered, especially since she is 4 now, it would be best. Uterine cancer is very very common in bunnies around 4-5, not trying to scare you, but it's something to think about. I hope she'll be ok