Can one use an external Android or iPhone device with the Phantom 4 Pro+ (with builtin display)? For example, by attaching to USB port on remote? Possible or no? I just got my 4 Pro+. Might be nice to use a larger screen sometimes.

Its already answered in FAQ-s. No, you cant. However, as far as i understand (sorry if i am wrong, i am beginner too), you can connenct any device with HDMI input to remote conteoller HDMI output, to see camera video on larger screen. But again, you cant use DJI GO 4 app from external device to controll your aircraft.

Not possible!