For a given prop company's make and model, yes, you want the prop to give you the proper max RPM at WOT and this is done by changing the prop's pitch... the right pitch can change when you change prop brands and/or styles... the pitch will definitely change when you go from a three to a four blade prop... In general you need to drop an inch or two in pitch which ultimately results in a lower top end speed... the real advantage of a four blade, is you can get and stay on plane at a lower speed... you need to decide what speeds you use most often and pick the prop that will work best for you... my Marlin likes a 3-7 mph troll or to be cruising between 22 and 35mph... anything in between or above it's hating life or I'm hating my wallet... I don't care if my top speed drops from 42 down to 40 as the fuel burn is obscene at full throttle... the other reason I'm looking to move to the four blade is for the extra grip... I lifted my motors one hole last year... they were buried deep below where they should be and I couldn't come anywhere near the speed and RPMs I should have... raising the motors definitely helped the boat run and handle better, but the props will occassionally slip on a turn or on an unexpected wave or boat wake... the four blades should help with that...
Does it make sense to change props to recover the cost of buying them??? Yeah right... not a chance!!! but if you need new props (both of mine have a small crack in one of the blades), yes... and get the right one for how you use the boat... whether it be a three or four blade prop... for what it's worth, I've never seen a complaint from someone who has made the switch to four blades...
