The disc based version of thr PS5 is only backwards compatible with certain PS4 games. It will play most of them, but each one will perfotm differently in terms of quality. For example, I have heard that Assassin's Creed: Syndicate is not supported on the new system. Tge game kind of runs, but it has a flickering black bar effect that plagues users as they attempt to play the game. With this said, I would say that about 98% of your PS4 gaming library will likely work properly, though there maybe some exceptions.