The BBC has a cute story about the man who supposedly invented the ATM, and the reason for a 4-digit PIN:

One by-product of inventing the first cash machine was the concept of the Pin number.

Mr Shepherd-Barron came up with the idea when he realised that he could remember his six-figure army number. But he decided to check that with his wife, Caroline.

“Over the kitchen table, she said she could only remember four figures, so because of her, four figures became the world standard,” he laughs.

Could only remember four figures? Is that not grounds for divorce?

But seriously, I find it strange the reporter wrote “Pin number”. I read that as the personal identification number number, which makes me suspicious of the research that went into the story.

So the PIN came first, or later? Would be interesting to know what event actually prompted the need for a PIN.