You could have cavitation with a 4 blade because of blade loading, that the 5 blade could eliminate. Just remember that a fixed prop is only right for a given rpm, it is wrong for the rest, meaning that if you had a controllable pitch prop you could adjust if for every condition and extract maximum efficiency. This best demonstrated by the amount of rotational force needed to spin a prop at a given rpm. It may only require say 75 hp to spin the prop at 1400 but your engine is actually capable of delivering 200 hp, you would be under propped by 60%. If you could adjust the prop on the fly you could get a lot more thrust at the same rpm and be a lot more fuel efficient and less wear and tear on the engine. Using a fixed pitch prop we have to prop for max rpm and not overload the engine, and this makes us extremely inefficient from idle until we reach max rpm.