docs: def prompt_4_mfa_serial(input = None):
      """
      Prompt for an MFA serial number
  
      :param input:                       Used for unit testing
  
      :return:                            The MFA serial number
      """
      return prompt_4_value('Enter your MFA serial:', required = False, regex = re_mfa_serial_format, regex_format = mfa_serial_format, input = input)