I wondered once about this, but it kind of make sense from the point of view of usability.
Unlike any webservice, you usually have very few attempts to make a successful login before getting locked out, so even if it's four digits, the odds of a successful brute force attack are very low
Unlike any webservice, you usually have very few attempts to make a successful login before getting locked out, so even if it's four digits, the odds of a successful brute force attack are very low