That can be problematic because there are 'only' 10,000 possible combinations for a four-digit PIN (based on the numbers 0-9).

With a four digit PIN, there are a possible 10,000 combinations. But with a six digit code, there are 1 million possible combos, making it a lot tougher for someone to crack your security code.

Apple to require 6-digit passcodes on newer iPhones, iPads under iOS 9 Stronger passcode ups the ante: there will be one million possible permutations.