Think of it another way, if this title were on PC, the PS4 would be something akin to low/mid while the PS5 would be something along the lines of High/Ultra. But the gulf in graphics presentation between PS4 and PS5 is larger than what we see in PC games between low/mid and high/ultra. That's because for something like Ultra on PC, the developer is mostly just boosting existing rendering effects and occasionally swapping in alternative rendering layers (like RT) for an established rendering layer (lighting, shadows, etc.), but they are almost never actually adding anything.
It is quite often easier to remove something from the lower quality version of a game than it is to add something to a higher quality version of a game.
Games on PC usually target lower spec hardware and then attempt to scale up. This leads to not a huge difference between low/mid and high/ultra other than resolution, asset quality and rendering quality. You will almost never see new things added.
If you instead target high and then scale down, you can not only scale down the aforementioned resolution, asset quality and rendering quality ... but more importantly you can just flat out remove things that don't fit into the rendering budget on the lower spec. hardware.
An example of this is that entire parts of the background scene geometry are just removed in the PS4 version compared to the PS5 version. Easier to remove those after the fact when you're trying to figure out what isn't necessarily needed to have a similar experience on the lower end hardware. OTOH - if you find yourself having to add entire geometric assets into a scene, now it becomes more difficult to figure out how to add in scene changing geometry without disrupting the feel of the scene that was originally created for that area of the game.
Not impossible obviously. But it's far easier and less time consuming to take an established scene/area/level and selectively remove things while keeping the same overall look and "feel" versus adding in things and still maintaining the same overall look and "feel".
Think about R&C and the remake of Demon Soul's, they'd be relatively easy to scale down to PS4 if the studios wanted to. However, the flip side of creating them originally on PS4 and then having them scale up to become their current PS5 versions would have been nearly impossible.
Or hell, think of any PS4/XBO title that has been scaled down to work on the Nintendo Switch. Scaling down to NSW hardware is possible and easier than trying to scale up an NSW title into a PS4/XBO game of the same scale and feature set as when you scaled down a PS4/XBO game to NSW.
Would Skyrim or No Man's Sky on PS4/XBO be what they currently are if instead they had targeted the NSW at the start and then later were scaled up to run on PS4/XBO? Those games would have been drastically different if they had started out targeting NSW.
Did targeting PS4 make it impossible to scale No Man's Sky down to NSW level hardware?
Regards,
SB