Unfortunately nobody make 4 blade #6 propellers as standard offerings anymore. Mewrcury stopped making them in 2006ish.
All of the old inventory we used to have has been sold for the same application as yours, ski racing.
We made a fair nimber of the lower pitch sizes back in the 1990's but not nearly as many as the upper 20's to low 30's. Keep looking for any diameter in the 24-27 range as we can adjust the props from there. We have all the templates to do this.

Brett