Bubby,
Hard to say exactly, but I've hit the mark almost every time I've gone from a 3 blade to a 4 blade of the same manufacturer by dropping it down one pitch. In other words - if he's turning a 22 pitch 3 blade, I'd probably try a 21 pitch 4 blade.
It also depends on if he is staying with the same prop manufacturer. If he were going from a Tempest to a Trophy (3 blade to 4 blade - both Merc props), I'd say 21 is probably his number. However, if he were going from a Tempest to something like a PowerTech!, I'd probably air on the side of 2 pitches down and go with a 20, since PowerTech! seems to pitch almost a half size up in the experiences I've had with them.
There is no set rules though. In all the testing I've done on my boat, I can turn a 26 Mercury Tempest, 25 Mercury Trophy +, 25 PowerTech TRO-4, and 24 Mercury Fury at max RPM with good to great results. That throws any "rules" right out of the window.
All the best,
Glenn
