It's one of those things that works amazingly, but it may not always be safe. Hydroquinone side effects in lab rats included cancer. I have read that they were given thousands of times the normal dose and that's why they developed cancer, but I don't know if that's true or not. I would be careful using hydroquinone cream in a 4% concentration long term. As Robyne said, make sure you do it under the careful supervision of your doctor.
I know many people that have had success using hydroquinone 2% instead (which is non prescription) but if your hyperpigmentation is severe, maybe that won't be strong enough.
I guess the best piece of advice I can give you is to just carefully follow your doctors orders while using it. Whether or not hydroquinone side effects include cancer in humans, I don't think it's a wise idea to use an artificial substance like that long term. Remember, you don't want to turn out looking like Sammy Sosa