What do the 'X' and 'K' codes mean for rubber bands?
The prefix 'x' means that the element design-ID used by LEGO is not known or was not known at the time it was entered into the Peeron database (www.Peeron.com).
If a design-ID is known, the people who maintain Peeron use that to identify the piece. Otherwise you get 'x' with some number...
This is indeed often the case with strings, rubber bands, gaskets as there is no room to print the design ID on the element.

I have no idea about your 'K' prefix, could you give a reference where 'x89' is refered to as K4 ?