If there's no other error, this is most likely due to a small piece of plastic that's broken off inside your iPhone 4. This is not at all uncommon and is due to the little plastic thread being deliberately delicate. It's basically supposed to break off as soon as you try to unfasten the screw it holds.
Don't worry about it, you can simply remove the small part, the rattling will stop and your phone will work perfectly fine without it.