Ummm, it's likely perfectly true that they only recently decided to back port to the PS4. It's entirely possible that this was originally meant to be a PS5 exclusive title. Which means that their hardware target for the game was the PS5. However, when someone at Polyphony saw how easy it was to make the game run on PS4 by just reducing or removing some rendering effects, it becomes a case of, "Why not offer a PS4 version?"
Basically a prime example of the existence of a PS4 version having absolutely no impact on the visuals of a game targeting the PS5.
Or you can just assume that Sony and Polyphony Digital are liars.
While I have no idea about Sony, I just do not believe that Polyphony Digital are lying about this considering they have always been one of the better studios at maximizing what they can get out of the latest PS platform ... even if they are taking increasingly large amounts of time in order to do so.
IMO, this was always going to be a PS5 exclusive title and the game was always targeted at the PS5 hardware. Thus the only reason a PS4 version will exist is because it's so easy to back port a title to PS4 from PS5. Alternatively, there's a PC version being developed simultaneously, so why not use the lessons learned from PC development (scalability) to easily back port the PS5 version to the PS4.
Alternative alternatively, Nixxes was tasked with porting it to PS4 and PD has nothing to do with the back port. This might be the most credible take on things. And this would again mean that a PS4 version of a title has absolutely ZERO effect on a game being 100% optimized for the PS5.
Regards,
SB