I'm pretty sure KH4 will be PS5 exclusive like Forspoken, FF16, Foamstars, ect.. Sony has its pick of the litter with Square and will always pick Square's best games as exclusive this is why they dont need to e en buy them.
That may've been the case prior to the new S-E President coming onboard, but may not be the case going forward. I.e getting those games as timed console exclusives could cost them more, or may be off the table altogether.
There are many benefits that an acquisition of Square-Enix could get Sony that simply doing these timed exclusive deals will not. However, making significant share purchases and financial investments into Square-Enix can get a lot of those same benefits while avoiding the tribulations of review processes for an M&A.
The irony here being, I feel this is something Microsoft will try doing going forward if/when they acquire ABK, and they've already started in a way WRT Japanese 3P like Sega/Atlus. Better for Sony to get ahead of them on that front in a more expanded capacity than wait & be reactive. But, seems like waiting & reacting regarding these sorts of things is their MO :/