This is were security meets the real world. The number is not secret but people have been taught to keep it confidential. And when you know the last 4 digits social engineering har become a lot easier.
But since then I have experienced how scared mugglers get when they get a threatning mail with the only legitimacy of naming and old leaked password.
This will be easy to exploit on a scale.
Scammers used to prey on the weakest hence the many Nigerian Princes. But as they get more sophisticated and move up the chain they start to look more and more legitimate.
