I went from the 27" ProArt to a 32", both 4k (PA279CV to PS329CRV specifically).
The PPI change is drastic, I maintain a rather short sitting distance (probably less than 2' / 24"), the monitors are on hydraulic arms and sometimes I pull them close enough to be above the keyboard.
IMO the PPI on the 27" is "fine", it is not retina but very rarely do I find text or smaller UI elements rough to see. But on the 32" the issue is apparent. I guess to my eyes the sweetspot threshold is somewhere between 130-170PPI. But I still stick to using the 32" because the extra real estate can enable a larger UI res, I am currently using 3360x1890 UI in macOS display settings, which allows 3 vertical windows side by side without each being too narrow. Using this UI scaling on a 27" would be too small.
To me, a 32" 5k if not 6k will be the holy grail / end game.