Don't worry too much about the development of KH4, it shouldn't take too much time. Maybe if there is something that delays the development a bit it should be due to the shaders once they upgrade to UE5, i haven't touched UE5's source code so i don't know how the lighting with Lumen works but since they used their own shaders for KH3 for different worlds, they probably will do the same for this one(Nomura confirmed in the Q&A that there are different worlds in KH4 too).
Plus, Nomura said that he was indecisive about making KH4 or KH Verum rex but in the end he decided to go to KH4 alone because 2 games for the consumers is a bit too much + a lot of people were worried about Sora so KH4 is the next big game without spin offs in the way(tho Nomura also said that Verum Rex's development name is "Verum Rex" so maybe there is a team starting with the project).
Anyways the Osaka team already knows how to properly use UE4 so even if they change to UE5, it shouldn't take too much time. My hopes are for 2023-2024