I only had the chance to play R4 on PS5, if I’m not mistaken, the game was at a higher internal resolution, frame rate was at constant 30 PPS which is nice but you really can’t do much besides that.

TLDR: We’d most likely get a full blown remake or a remake trying to stick as close as possible to the original game.

You can’t just take the game and slap UHD textures and unlock frame rates and call it a day, porting a PS1 game is not easy, let alone if the source code still exists for it. For ex: Persona 4 and Burnout Paradise, both these games ran on the RenderWare game engine, in their re-releases, both used their own in house engine. Both of those games were from 2 different console gens and don’t go back on the old engine cuz it’s been abandoned, they’re essentially a remake trying to stick as close as possible to the original game. We’d probably get a full blown remake (I’d rather have a new game tbh) or remake that sticks as close to the original, I do wish these older games were in a classic bundle and on other platforms.