How safe is it to make public the last four digits of a credit card?
Credit card numbers have a specific format. Digits tell you what type of institution issued the card, what bank issued the card, the account number, etc. The whole 16 digits must conform to the Luhn formula, a simple mod 10 checksum.
Attackers have lists of valid first four digit numbers (which can be narrowed down using other information often provided with last 4; eg country). Is it feasible for them to brute force matches to these first 4 and last 4 digits using Luhn and fast computers?