As pin() has no inputs

I'll admit to using "black box" in a poorly defined way -- what I meant was that the function in question isn't really random -- it transforms well defined inputs into outputs. In this sense, pin() takes no arguments, but it does have inputs -- four calls to rand. In my view, the right approach is to vary the inputs and examine the outputs -- explicitly test boundary conditions of known inputs rather than trying to evaluate it probabalistically.

If you rely upon knowledge of the implementation to not bother checking for length 4, then a modification by someone not understanding the significance of "%04d" that omitted the zero would not be detected.

I was referring to the original loop version, actually. It will always concatenate 4 integers. That doesn't vary based on the input. So while it needs to be checked, it doesn't need to be checked everywhere, we only need to check a situation where an input might produce an integer of more than one digit.