If they are buying an Ipad(s)... when the system ask you for those 4 digits always take a card in your hand, don't ask them to tell you. EVERY day I have people with fake cards, they have IDs matching the cards and even if you enter four digits from a card it will go, but the card is fake. Its good to check the card, like the quality of the print. If the card looks ''cheap'', just call GSTL to double check.
When I see those cards, I just put some other last four digits and it will not be approved, and you see on them that they are not surprised. You can recognize them when you ask them for attachments and they will anything,... you can see they don't care about the price (because its not there money). 😀