It's already a great game graphically and
it's technically a PS5 game because when GTSport was in development, they were already using a PS5 Dev kit.
In theory as far as graphics go, its what you would find on a PS5 to a degree but because its on a PS4, the only downgrade because of cross-gen is no ray tracing.
There's other things then Graphics that use up memory and processing power, Physics been one of those underlying items. To get the Physics of ACC to work on the PS4 they had to sacrifice FPS, Graphics, Replays length, lobby sizes, among other things.
It's going to be a beautiful looking game on PS5, but theres no Ray tracing in the race on PS5, kaz said this , and you don't really need it in a racing game, Ray tracing in scapes.
The timing of the PS5 started development at the sametime as GTS, thing like the memory, and processor would not of been finalised until way after GTS came out. they did not use a PS5 dev kit. They probably knew what the General Plan for Ps5 was but there was no dev kit 5/6 years ago.
Their will be things that ended up been shelved to allow the cross play, The game mechanics according to Kaz are the same on the 4 as the PS5.
It was going to be a PS5 game then the money people stepped in, which would account for the delay,