@shredman
You might have a look at this tool/script:
https://retropie.org.uk/forum/post/6396

It should be exactly what you need to ensure the PS4 controller always stays the master controller.

also a n00b here. trying to do this and i'm getting :
"Your local changes to the following files would be overwritten by checkout: retropie_packages.sh
Please commit your changes or stash them before you switch branches"

No idea what to do here...please help. thx

@salvaliebana install joystick selection. Go into retropie menu. If you see joystick selection then it is already installed and go to paragraph 2. If it is not read paragraph 1.

In the retropie menu, go into runcommand. In there install joystick selection. Exit. Restart your pi.

Go into retropie menu, then into joystick selection. In option number 1 you can select which joystick is each player.