and said he got to 4000 RPMs "way to easy"... he told me if I got it proped "right" I would gain ~5-6 mph.
What does 'way too easy' mean? If it was WOT, then you're 100 RPM off maximum. If 4000 wasn't WOT, the number (by itself) doesn't mean much other than indicate a potential prop change needed. You're 100 RPM away from maximum (if actually WOT) and he says you can gain 5-6 mph? I bet he sells duoprops. Have you tach checked, then verify for yourself what the RPM gets to (without over-speeding too much), and know what your loading is. If you're light, or maybe medium loaded, I certainly wouldn't fool with 100 RPM. If you're heavy, then think about a prop change. V-P says about +/- 250 per prop set increment, but I doubt a change to A5 will get you 5-6 RPM, A3 is going the wrong way. If you have throttle left when you get to 3900 (how quickly you get there is irrelevant, it's how much throttle you have left, if any, that counts), then a little more investigation is in order. BTW, I don't believe prop slip calculators mean much with duoprops ...