Mask all but last four of card number when unfocused

It could be a handy anti-over-the-shoulder feature to mask all characters but the last four of the user's card number when they unfocus the field. This lets them still see their entire card number when typing or modifying it, but also means when they move onto the next field, we're showing less sensitive data on the screen.

I've seen this pattern elsewhere, but I'm not 100% sure where. I like it better than treating the whole field as a password field because it's much easier to see and confirm the number.