Not sure how you reconcile frontier abandoning the Playstation "long ago" whilst they are actively porting Odyssey to PS4 right now. Sure, console-specific bugs don't get any attention at the moment, but loads of those bugs are now irrelevant as they won't even exist in the current build. You will instead have shiny new bugs you haven't seen before.
I'm actually curious about this myself. I think that the Cobra engine needed updating to support new graphic tech in Odyssey, but I'm not 100% certain on that. If the Cobra engine itself (the "JRE") needs to be updated, then Frontier would have to reassemble at least some of the team (or assemble a new one) that originally ported Cobra to PS4 all those years ago. The good news is that if this is the case, they might actually fix some long-standing Horizons bugs and graphical issues while they are in there.
The other option is that Cobra remains the same (the equivalent of using UE 3 for both Horizons and Odyssey), in which case it's all Cobra programmers rather than PS4 programmers doing the work, and they are basically turning knobs and flipping switches until they get the best performance-to-quality ratio on PS4 (kinda like I did with my PC settings to get VR to work). IMO this is the less optimal approach, because the best optimizations happen in the low-level code of the Engine, and Cobra on PS4 is pretty dated if they are still using the version that ED 2.3 launched with.
Of course if they ARE rewriting Cobra for PS4 to optimize for the new lighting, textures, physics, and planetary generation, why not go the extra mile and flip the few extra switches (big assumption on my part) to also make ED run on PS5?