Nick,
Notwithstanding the weight gain, wouldn't the addition of a fifth blade allow all the blades to be made closer to the ideal shape for hovering (long and narrow), while preserving the solidity necessary for high-speed and maneuvering performance?
Additionally, wouldn't the addition of a fifth blade allow all blades to operate at a lower angle of attack, thus increasing the retreating blade stall margin?
As to the vibration, do additional blades actually reduce the energy of the vibration or just shift it to a frequency which is less objectionable?
-Stan-