It depends on if your concern is failures due to reentry or blade induced torsional vibration.
Reentry failure is related to blade area. In general, the more blades you have the greater the total blade surface area. It's possible that a four bladed prop would be worse than a five bladed prop if it's blade area is greater.
With regard to torsional vibration it is a good generalization to assume that vibration levels decrease with the number of blades. Higher frequencies are more easily damped and there is quite a bit of damping in a marine propulsion system. You have to remember that driveline vibration is a function of the whole system. Outdrive type, drive ratio and coupler type all come in to play. In some cases fewer blades are better.