I have been told the four blade would be more efficient.
According to the prop shops we've talked to, as well as people in the marine engine inustry we know, the fewer blades a prop has the more efficient it will be. The most efficient prop would have one blade but it'd be a bitch to balance
* So assuming the same diameter propeller and optimum blade design, two blades will be more efficient than three blades, three blades will be more efficient than four blades, and so on.
But, the more blades there are, the smoother (or less vibration) the prop will be.* And there may be some thrust advantages with a higher number of blades.* But in terms of basic efficiency, a three bladed prop will be more efficient than a four bladed prop of the same dimensions and turned at the same rpms.
Also, a three-bladed prop will back better (less propwalk) than a four bladed prop of he same basic dimensions.
We were going to change from four blades back to three blades (which out boat had when built) but when we learned our four bladed props were still good we kept them and had them completely reworked and repitched down.* Saved us several thousand dollars over going to new props, but the four-bladed props are not as efficient as the three bladed props would have been.* However they generate more propwalk which in a twin-engine boat is an advantage when maneuvering.