Found a Zinger 24x10 and it did help by providing an additional 600 RPM. My 4.2 is up to 5600 RPM, still short of spec and what you are achieving. Have not yet attempted to adjust the mixture settings with this new prop running at the higher RPM. Since these settings were last adjusted using props turning at slower RPMs, I suspect I may have some opportunity to gain a few more by not assuming the settings at the lower RPM are good for the higher RPM.

Its already getting quite cold to stand behind a prop, up here in Michigan.

What fuel mix ratio are you using on your 4.2?