Like ballast...it depends.
So far the biggest reason I have heard is loading and limited by diameter.
I have never heard that 4 bladed in backing were a big deal. The efficiency of fewer blades is usually noticed up the rpm/load range I thought...so if anything...the 4 and greater numbers may be more efficient. Smaller outboard boats I thought used 4 plus blades for "hole shots".....immediate max thrusting...so that would lead me to believe more thrust at lower rpm....not up the rpm chart.
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