As of now if one likes Bitwarden to generate a numeric PIN to be used for bank instruments the only way is to make the password generator opt all numeric but there is a limitation. Password generator restricts minimum 5 characters whereas most banks worldwide are still with 4 digit PIN.
Though a quick argument could be to have Bitwarden generate a random 5 digit string and then ignore one of the digit I think this manual process might have repeative duplicate PINs in a vault and same PINs across cards could be a security issue too. Additionally instead of manual process if Bitwarden itself generates a random 4 digit string it would be a good feature.