I'll buy the PS4 version since I feel like this game is mostly story and I'd rather not be spoiled. The graphics look good enough and honestly, the PS5 version wouldn't be all that different unless they spent a lot more time with it. These games already target 4k on the pro systems with the checkerboarding and whatever else so it's not like textures and stuff will really be substantially different. The models won't be different and the lighting would have to have been done with ray tracing in mind to really benefit alot (you can see alot of RTX stuff on PC that's an after thought not really being too impactful but games made with it in mind upfront looked better) not to mention performance on ray tracing is rough so I don't think its something that'll be simple to just toss in (im assuming the implementation is some low end stuff but that's unknown).
So yeah all in all, I think the only difference would be better framerates and native resolutions on top of stuff like draw distances, model count (model count likely won't matter much for the kind of game this is), and maybe some additional effects. They aren't delaying it to make the PS5 version. They are delaying it to complete the PS4 version. Took RDR2 a year to come out with extras that were worthwhile for PC and I expect Sony to be just as slow about it if they are making it at all. A patch would make more sense to me. I don't think PS4 games are going to be remastered much since this thing is supposed to be backward compatible so if everything works out well, games will retroactively have better framerates and resolutions like how the Xbox One X does if the game supported it and patches for those that didn't.