It happens to a lot of them. I have a regular p4 and I too have the stress cracks on the back two motors. I bought a new shell, but never installed it. I also fly planes and mini quads, so I fly the phantom pretty aggressively sometimes for fun. But, if you fly it like a Lamborghini, you're going to get stress cracks in the back. For those that baby theirs, they probably won't get them, but if you fly aggressively in sport, they're bound to show up. I haven't had an issue in over a year with near daily flying after discovering them. But, if you want to send your bird off in the mail for weeks or months to get it fixed, do that - but when you get it back, if you again fly it like you stole it, expect them to reappear. I've done some pretty hardcore testing on mine before deciding to just leave it, like attaching a camera to the landing gear and filming at a very high shutter speed to be able to slow it down and assess how it's handling and if there's any danger in continuing to fly - from what I can tell (and from the last year of experience), mine is safe to fly. The shells are super flimsy.