I unboxed my p4p for the first time and am trying to update the firmware. The control has already updated but the aircraft is having problems connect via USB cable. I keep try turning on/off control/aircraft/app in different order with no luck. It keeps saying "connect aircraft". Could it be a faulty cable?
Have you tried using a computer with the Assistant 2 to upgrade the firmware for your P4 Pro? It is real easy that way.
Your description shows a clear disconnect between the RC and the AC. There are two causes for that. The two are not linked and one or the other is not using a compatible firmware. Try the assistant 2. It is downloaded from the P4 Pro downloads page.
Just to clarify, you upgraded it through DJI GO 4, right?
Please upgrade the aircraft with Assistant 2 as Cetacean mentioned, if it cannot be upgraded, please post a screenshot here with the error message.