Mactec54, again, thank you for the info and contributing to the thread. This is good info for lots of people so please get that I'm trying to fully understand. I don't even own a CNC or a spindle or VFD. I just want to understand.
I totally get that equipment needs to be properly grounded. I have a general OSHA safety cert and I understand how lethal high voltage (even low, actually) can be. SO no argument there.
I didn't fully appreciate the need for the ground to be shielded, but it does make sense that it would be subject to EMF and other noise, so no argument there either. For my future CNC I had also planned on just measuring the connector and buying IGUS cable that does fit (and has all 4 conductors). So I get it.
One thing is still a bit out of place for me. I DO understand the issue of CMV and the damage to bearings, etc, that the voltage having to jump across bearings and bearing races can cause. BUT... almost all of the chinese spindles come delivered with the 4 pin on the input connector connected to absolutely nothing. Literally a complete open circuit. It's not attached to the outer casing, nor to any brushes that might help properly dissipate internal CMW. Many users don't know this, so they do nothing, which is probably the most dangerous action. The only safe-ish option (I think) is to open up the spindle and attach that pin to the inside part of the casing. That does not help with the bearing problem, but it does at least ground the spindle housing. THIS is what I was referring to.... that in this scenario, there is no difference between grounding that 4th pin and just grounding the outer spindle housing, because that's literally what you connect that 4th pin to anyway on the cheap chinese spindles.
I'd love for there to be a way to connect the 4th pin to some special brushes to properly handle CMV but I don't see how that's possible (is it?). Anyway, mainly making a point that the chinese spindles are "special" in this case that they have a disconnected 4th pin and you must choose what to do with is, and that I don't personally see a way of doing anything with it that's better than using it to ground the case/housing.
Again, thanks for keeping the discourse here civil. I think it's important to keep these forums an actual intelligent resource of info, so thanks for clearing all of this up for us!