About 10 years ago I built a Midwest Mustang Sport .40. It had a 54" wing span, a "Fox .40" 2 cycle and flew great with a 2 blade 10-6 prop on 10% fuel. I went to a 2 blade APC 9-7 prop and 25% fuel and it screamed!!!! I flew this plane often for 4 seasons and killed it the first flight of the 5th.
A little off the subject... This was the first plane I ever built or flew (not counting Cox control line) I learned to fly it on a buddy box at the field in Oyster Creek TX.
As far as the 4 blade prop, I'm now building a 102" Nosen P-51 and plan to use a 2 blade on it. To me, it doesn't seem worth the trouble to swing a 4 blade prop unless one is going "all the way" with the scale look.
I hear myself talking,,,,,, all the while, I'm planning on putting a scale cockpit in it....................