One of the many factors that I've been told (and backed up by my machining experience) is that in the beginning of CNC machined props, the fixturing and wild 6 axis toolpaths to required create a 3 blade prop (where the leading edge of one blade overlapped the trailing edge of the next) was not feasible to machine in one setup and harder to keep balanced. 4 blade props were easier to machine at the time because the blades did not overlap as severely. Since that time, advances in CNC machining techniques have made machining the wild features required to have overlapping blades on a 3 blade prop more economical and less risky. A three blade has a little more surface area than the equivalent four, as well as a little more cross section in the weaker areas that will make the prop more rigid (performance wise) as well as more resistant to major damage. The "myth" that a 4 blade is more balanced still floats around out there from the "old days" (we're talking the late 90's here) of CNC props. I'd bet my beer money that I couldn't tell the difference of equivalently sized 3 vs 4 blade props on my boat though.

Either of these are light years ahead of cast props from the 80's like many of our boats came equipped with from the factory.