That is completly untrue. The optimum speed is determined by the tip speed, a multiblade prop has the same tip speed at the same RPM and diameter as a two bladed prop.

Nah dude, the prop speed only stays the same if you are at the same RPMs after switching from a 2 blade to a four blade. The 3 and four blades are less effecient than the 2 blades because when the 1st blade or a 4 blade propeller passes a certain point, then the 2nd blade passes right after it, it's riding right in the 1st blades wake and doesn't scoop up as much air as if it were a 2 blade where the blades a 180 degrees apart.