I have read many a thread regarding registering with the FAA but I haven't seen the answer to my question. I just bought a P4 yesterday and it was a used unit. How do I know if it was registered with DJI ( he says he didn't) and do I still need to do it and why? Also, how long do I have before I actually need to register it (and I know I should do it ASAP). But how long can I actually wait?