More of a problem when limited to 4 digits. Once you're beyond that you can start duplicating numbers. Say, for instance, you have a 7-digit PIN. Make sure you've duplicated two of them. So you could have a number like 1233455 (no, I don't have that number on my luggage, or anyplace else for that matter). Even if someone sees you've used the numbers 12345, they (probably) won't be able to see you used "3" and "5" twice. More useful on "smart"phones and tablets where you have smudgy fingerprints.