It seems that in the UK that banks have universally limited you to a 4 digit pin (6 digit in other countries)… and some don’t even let you change it…
AFAIK the standards allow up to 12 (ISO 9564 if my google fu is working today).
Obviously this won’t work with the prepay card but once you have your own banking systems, could you consider allowing us to set longer ones? With the caveat that if it doesn’t work on a particular device it’s our problem (big warning in the app or something).