The thing is, there are a lot of PINs that don't have to be 4 digits - the international standard for PINs is ISO 9564. I don't know if this applies to debit/credit cards (I just phoned my bank and apparently they only support 4 digits), but the standard does say that PINs can be from 4-12 digits long, and I remember reading about cards that have more digits, so I'm not sure what's up with that.
The reason I thought about it was because some time ago I was creating a voicemail system for use in an online game which was there solely for players to be able to hack it. The prompt it used for the players was "Please enter your password followed by the pound key". It was instantly assumed that this was 4 digits (and reported as such in the in-game forums), to the extent that players never considered that it might have been any other length. It actually *was* four digits and they managed to get it from in-game clues, but it could easily have been more. The fact that players never considered that it might be longer astounded me, and I wanted to follow up on it at some point.
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The reason I thought about it was because some time ago I was creating a voicemail system for use in an online game which was there solely for players to be able to hack it. The prompt it used for the players was "Please enter your password followed by the pound key". It was instantly assumed that this was 4 digits (and reported as such in the in-game forums), to the extent that players never considered that it might have been any other length. It actually *was* four digits and they managed to get it from in-game clues, but it could easily have been more. The fact that players never considered that it might be longer astounded me, and I wanted to follow up on it at some point.