I do not understand what you want for the x-tick labels.
First, run your code with this to see what it does, and then adapt this to do what you want:
set(gca, 'XTick',logspace(0, log10(20000), 5), 'XTickLabel',1:5)
In general, it is necessary to define both the tick locations themselves, and the tick labels.
The labels can also be text (character arrays) or strings as cell arrays. That is sllightly more complicated, so I will not discuss it now, however I can help you with it if that is what you want to do.