I worked in electronic banking for 23 years. The protocol behind bank cards can accommodate up to eight digits. In most countries they use six digits. In the USA, and Canada they only use four.
My employer uses a four digit pin along with my electronic ID badge to open the door during off hours. My former employer (in Asia) required six digits.
Microsoft requires only four.
Why?
My bicycle combo lock has six digits. Why, because I refuse to buy one with only four digits.
The PIN on my iPhone is set to eight digits. I have no problem remembering them.
Read more: https://iphoneus.org/faq/why-only-four-digits.html