The first four characters are what is known as the Bank Identification Number(BIN). Basically, it tells that Visa/MasterCard which bank issues the card. And how to route the transaction so it gets approved and posted. In order to be secure in online use virual credit card which is more secure for online transactions.
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The first number is 4 for all visa cards. Rest of the numbers are based on the issuing bank, the routing number for the bank, the account number etc.
The security code on any credit card would be the last set of digits on back of the card with is the three digit number
The first four numbers in pi are 3.141. Backwards, the first four numbers are 141.3. Pi has been calculated to over a million digits.
Usually, a visa card has 16 numbers.
The sum of the first four prime numbers is 17.