US Stove 6041i Manual says to use 4" if your altitude is over 2500 ft. I'm sitting just under 6000 ft. However, I can't find a CornVent™ kit in 4" diameter. I can find a duravent "multi-fuel" kit in 4", but from what I understand, those will still corrode if I burn corn. How big of a deal is it if I use the 3" kit instead of a 4"?