I don’t think anyone really answered the OP’s question, “Why do some boats have 4 blade props and others have 3?” The fewer blades the more efficient the prop. (Racing airplanes have been known to use one-blade props.) However, the prop also has to be able to handle the power from the engine. For a given diameter more blades are able to transmit more power to the water, albeit at a somewhat lower efficiency. Two blades would be the first choice. With a bigger engine, if there is insufficient room for a larger diameter prop then add more blades. Blade pitch, shape and position are important details best left to the experts.