Why is the last number in the SMS of China Bank the fourth after the mobile phone number?

The last four digits of a bank card are not unique, because three of the last four digits are serial numbers, and the last digit is the check code, which means that the serial number and the check code are the same. However, the complete card number of the bank card is indeed unique, and its uniqueness applies not only to the issuing bank, but also to all banks. If the bank card tail number is the same, it will not affect the user's use. The bank uses the complete number of the bank card to identify the identity of the user and the card, and the same tail number will not make the bank get the information wrong.