Bit worried here, Dave - i realised reading the page that you might not be joking?
Your 4-blade designs are fine. The reason to avoid 3 blade is that the blade 3/4 wave resonance would match the rotor 3P frequency. This means that if there was an upthrust at 90 azimuth say, all the blades would resonate in synch hence transfering 3P vertical mode to hub and thus fuselage. Comanche went for 5 blades, i believe, since it was found that the 3/4 wave resonance might match rotor 4P frequency (blade stiffness pushes up resonant freqs).
The reason blade 3/4 wavelength mode is a problem is that blade can be seen as a flexural structure, fully constrained at one end and free at the other. This is an over simplification but means that blade rotational frequency, for effective hinge offset calcs, will be 1/4 wave mode. It also means there will be odd order modes above this, 3/4 5/4 7/4 etc. That is how i did the dynamic guestimates above. Naturally having internal damping, or active tip damping also helps to minimise vibration at source.
Coaxial package protects for any number of blades required, so is a good choice. I would imagine 5 blades and above is getting risky on an
intermesher...
Mart