Imagine releasing a game on a hardware that is 8 years old, simply thinking about this fact is underwhelming. Just look at Forza Horizon 2 on xbox 360 for a really good example.
So simply some things will not be there for GT7 on ps4, because seeing how GTS on the same platform has pop-ins (little, but still), no TOD, reduced LOD on cars on track, some 2d trees and etc. So there is no room for improvement at all.
-Ray tracing, simply an ad for a new consoles, i think maybe they'll include it in cutscenes, menus and etc, but generally I haven't seen it in a gameplay footage trailer, maybe it's too early for that. But it allow for cars to look even more realistic and then maybe at rain we can finally beat these "GTA V 8K RTX GRAPHICS"
-It'll take too much time, developing two versions, different LODs of tracks, cars and etc, it'll just take time and knowing PD's nature with GT titles it's just a crime to do that.
-Every newer generation GT title improved on physics by some amount, so cross-gen just might be a copy paste from GTs, maybe with some little tweaks, they can't do different on PS4/PS5, because multiplayer so there is a drawback, again.
-SSD, simply SSD.
-Dynamic weather, time. Kaz himself said that GTS didn't have it, because Ps4 was too weak for it to maintain 60fps, so here is another long awaited feature will be gone, because cross-gen
-New generation graphics, better trees, environments, which where maybe fine in GTS at some point, but again there is just no room for improvement in GT7.
-PS5 utilities new method of shorting space on games, by not doing different LODs. I can't imagine how much space will take GT7 on ps4, with let's say 600 cars and maybe 10 more tracks then in GTS
-Dualsense features
-3D audio features
So whoever wishes GT7 on ps4, I'll literally develop a time machine and then replace Kaz and develop GT5 on ps2 and will give that version to everyone who wishes cross-gen.
My opinion would be release a paid dlc for GTS, for like maybe 10-15 bucks with some new tracks/cars. GTS has sold around 9kk copies, so they'll earn something from it.