Unfortunately being the newer game didn't automatically make it better, PFM was bought out by Ubisoft and Ubisoft made P5 and they didn't make a *better* sequel per se haha
Imo these are the reasons that people *may* choose P4 over P5:
~Comfort or nostalgia, having played P4 first
~P5 has more bugs/glitches as it is a less stable game build
~Posing petz for showing is harder in P5
~Although P5 added litters, they don't actually increase genetic diversity, where as breeding multiple times in P4 does.
~The playscene graphics (that 3D look they attempted) is disliked by many
~The playscene editor is more broken (doesn't allow images with transparency for example)
~There is no playpen scene in P5
~Even harder to feed your petz due to bouncy bowl glitch
~The minigames (esp before tinker) were annoying in general and having to do them to get toyz
~Many of the new breeds were made very lazily, basically just slight modifications to existing breeds and not thoroughly tested for breeding etc.
~The lack of toy closet/carry case as the closet - adding this to the game actually prevents you from being able to do certain things, like take flowers to another playscene.
~A lot of content creators play P4, so they first and foremost make content for P4, meaning there is slightly less content available for P5.
I do like the breeds despite some issue with them, I just used fixed versions of the P5 breeds in my P4 game. I do miss the weather and day/night cycles, I thought that was nice!
I don't play P5 though so I can't quite remember any other pros haha And ofc people play P5 for their nostalgia of that game too, and a lot of the things above may not apply to them (they don't show, they like the graphics, they don't care about the lack of playpen or whatever).