Since a Visa card always starts with a 4, that's actually 5 digits you have to have in common between two cards.

How likely is that???

(I know I've never yet had anything even close to that with a Visa or MC, and have only come close with cards issued by Amex, where I seem to often have only one or two digits -- of the last 5, not just last 4 -- different between them, as if they have a sequence of last 5 card numbers predetermined just for me personally. But since Amex-issued cards are always from the same issuer, I presume they have a system to make sure you never end up with two cards from them that have the same last 4 or last 5, as then they themselves would have problems telling them apart.)

And I don't think you can request a specific set of last 4 digits from any MC/Visa card issuer, can you?