A little over 50 years ago, John Shepherd-Barron was taking a bath and thinking about a machine that could dispense cash instead of chocolate bars when, he claims, he came up with the idea for the ATM. Realizing he needed a way for the machine to safely identify the user, he then suggested the PIN.
According to the BBC profile, "The Man Who Invented the Cash Machine," with his old army number in mind, he originally settled on having a six-figure personal identification code. But his wife, Caroline, rejected the idea. "Over the kitchen table," he told BBC with a laugh, "she said she could only remember four figures, so because of her, four figures became the world standard."
Caroline's forgetfulness has made millions of lives more convenient, but it has also exposed users to some security issues. Instead of one million possible PIN number combinations, hackers only need to consider 10,000. This substantial difference is what inspired Apple to pivot to the six-digit passcode when they released iOS9 for the iPhone.