PS4 controllers cannot be used to play PS5 games on a PS5
PS4 controllers can be used only for PS4 games on a PS5
Turn on the PS5
Connect the PS4 controller to the PS5 with a USB cable
Press the PS button on the PS4 controller
Select the user profile if prompted
Disconnect the cable if you want to use the controller wirelessly
To reconnect wirelessly, press the PS button again when the controller is already paired