He probably just misremembered a few characters of his password, like mypass1234, mypass1235, mypass4321, etc. It would be time consuming for human to try all

He probably just misremembered a few characters of his password, like mypass1234, mypass1235, mypass4321, etc. It would be time consuming for human to try all the 4 digits but trivial for computer to try out the 9999 enumeration. The complication is finding out the algorithm used and the parameters applied to the algorithm. Once figured out, he wrote the small program to brute force the small enumeration. This is a very good illustration of the need for long and non-trivial password; otherwise, it can be easily guessed.