Not sure from where you're getting:

['Db'] -> -4 steps -> ['A#'],

...because if you're referring to the example in the description, that one is:

['Db'] -> -4 steps -> ['A']

and that is correct. A natural is indeed four half-steps below D flat.

You would achieve G flat by descending four scale steps below D flat (in the key of Db major), but the kata is not dealing with scale steps; it's chromatic differences (half-steps) that are being used here.

Since you are posting as an issue, I would ask that you clarify exactly what the problem with the kata is?