We've always liked the way Android phones' handle the unlock screen. Simply create a pattern across nine dots by playing connect the dots, and use that to unlock your device. There are so many possible combinations, yet it's easier to remember than a number. Input is also fast. Also, this takes care of not only the unlock screen, but any pass code protection in one swoop, where as with iOS devices you have to 'Slide to Unlock' then enter your code.
Apple may be testing this function out internally with some of their applications. That is according to website 9to5Mac who has some sources within Apple. According to the website, Apple is currently using it with the employee Apple Connect App.
9to5Mac does say that it's unclear whether this feature will ever make it to consumer iOS devices. There is the possibility that it may just stay within the company and be used for employee applications such as those used by Apple Store employees.
It would be interesting to see this make its way to iOS. In our experience it's much faster for the user and relies more on muscle memory than having to remember a number. It's also more difficult to guess since most iPhone owners tend to use a number that is important to them, therefore easier to remember and more easily guessed.