@alfie14 : I didn't have to add PayPal details to get my subscription working. No idea why you would need to provide those details.
Contact Support: https://support.google.com/stadia and they will figure it out for you. It isn't like it wasn't public information that an active subscription would be required for 4K support. But if you feel hard done by, request a refund and go with another system of your preference.