webmisfit, I wouldn't put him down for his reasoning as it makes sense in a real-world context; this kata definitely could be built upon to incorporate data normalization, and such a kata, say a part 2, could also potentially make a good exercise for the practice of separation of concerns.

Zamay are you sure? its gone all test so you think you smart?

it will return ...#6 16

[space]16[space] is masking all but the last four characters which is what was defined by the specification.

But this code does not pass this test:

'4556 364607 9356 16 ' ... => "################ 16 "

But this code does not pass this test:

'4556 364607 9356 16 ' ... => "################ 16 "

ahahahaha
I also thought to do so , but then I realized that this is nonsense