With our model engines the biggest problem with 4-blade props is finding one that suits the engine/airplane combination. The lack of choice in 3-bladers and 4-bladers really makes the efficiency question insignificant. 3 and 4 bladers have a bad reputation more because they offer almost no choice, not that there is any real difference in efficiency. We simply don't have but one to try most times. No way to find the best 3-blader when there isn't but one that might or might not work. But we've almost always got 8 or 20 two bladers to sort through.