Step 1. Connect your controller to the console using a Micro USB Cable or Wirelessly by holding down the Share and PS Button for a few seconds and selecting the controller from the system's devices menu.
Step 2. Navigate to any game that has a PS4 version available and select the options button indicated by 3 dots.
Step 3. Select the PS4 version and you're ready to play.
The console will prompt you to use a PS5 DualSense controller if you're attempting to play a PS5 version of the game or there is no PS4 version available. Some games require separate downloads to access their PS4 versions.