Overwhelming majority of PS4 games will be playable on the PS5 say Sony in an FAQ. The FAQ has also listed 10 games which won’t be playable on the PS5 and are listed as PS4 only games which you’ll be able to see below…
PS4 only games:
DWVR
Afro Samurai 2 Revenge of Kuma Volume One
TT Isle of Man – Ride on the Edge 2
Just Deal With It!
Shadow Complex Remastered
Robinson: The Journey
We Sing
Hitman Go: Definitive Edition
Shadwen
Joe’s Diner
The FAQ also lists some limitations with the PS5 backwards compatibility such as the share menu won’t be displayed PS4 gameplay, however you will be able to use the Dualshock 4 controller with PS4 games on the PS5.
Check out the full FAQ here.