You could preorder the PS5 version. You get always both versions, PS4 and PS5. EU Store https://store.playstation.com/en-gb/product/EP2866-PPSA07618_00-TSW3STANDARDEDIT https://store.playstation.com/de-de/product/EP2866-PPSA07618_00-TSW3STANDARDEDIT US Store https://store.playstation.com/en-us/product/UP3004-PPSA07619_00-TSW3STANDARDEDIT
I think if you pre order TSW3 via the PS Store on your actual PS4 you will get the PS4 version to download. If you then get a PS5 you will be able to download TSW3 to that for free. That's how I understand it anyway.