I'm also considering a 4 blade...

...but here is what I have tried so far with results.

18' 73 Glassmaster Tarpon trihull 165 HP L6 Mercruiser I/O

Tested with two adults on board, fishing equipment and half a fuel load.

with,
13.75" dia x 21" pitch Mercruiser SS 3 blade; WOT 43 mph (gps) at 3,900 rpm, (bottom of recommended WOT rpm range) Trims out nice, rides smooth with good lift.

with,
14.5" dia x 19" pitch MW aluminum 3 blade: WOT @ 41 mph (gps) at 4,300 rpm (top of recommended rpm range) Pulls good out of the hole but seems to run heavier in the water. Prop blows out much lower in trim than does the SS prop mentioned above.

Question, would a 4 blade 14" dia x 20" pitch put me closer to the the middle of my rpm range, give me comparable trim and ride as the SS with possibly a better lower speed cruising ability, say 25 mph?

Stainless or AL?

Re: I'm also considering a 4 blade...

You may pose this question to WH, most of the guy's here are doing the I/O's are running 20 degree deep v's, now add a good stern lifting prop and you get less wetted surface reduced drag and good speed's. If the prop cant lift hull out far enough it would probably just slow you down.

Re: I'm also considering a 4 blade...

SS will be a little more efficient at moving as it has less flex than aluminum. Better hole shots although if you hit something with AL, it bends or breaks and you buy a new prop. SS will do more damage to your gear case etc.

Re: I'm also considering a 4 blade...

Alot depends on the hull design, as T.G. mentioned..."which" 4-bld you choose would also come into play.
I can also see someone doesn't believe in SS and thinks it may case gearcase damage....10-15yrs ago...maybe. Not today...you have as good a chance of ruining a lower with aluminum as you do with SS just because of the hub styles.
I would consider the BayProII or possibly an Apollo 4-bld for this application...both run real close in RPM in my test Data