The 4 groups don't have a meaning. They do, actually. First digit is 4 for VISA, 5 for Mastercard, 6 for Discover/Diners Club, 3 for American Express/Diners Club (those are shorter than 16). Also, first 6 digits for Visa and Mastercard are code numbers for the issuing institution.
Besides, what is the number of Visa card?
Are all credit card numbers 16 digits?
Take a look at your card and count the digits. You'll most likely discover that if you have an American Express, your card has 15 digits. Discover and Mastercard keep their accounts at 16 digits, and although most Visa cards are 16 digits, some can have as many as 19.
What kind of credit card starts with a 4?
Credit card numbers are created in a consistent way. American Express cards start with either 34 or 37. Mastercard numbers begin with 51–55. Visa cards start with 4.