I have only had my Z9 for about 2 months (50,000 images) and I am still learning every day how I want to configure the camera. I haven't noticed anything while sitting in my living room reviewing and changing settings. I have seen cases when I am shooting action and I decide to quickly change a setting and I press a menu button and the menu flashes on the rear display then the display goes black, I press the button again and the menu displays correctly. I have had to press a menu button 3 times to get the menu to display. I have always attributed this to the fact that I am in a hurry, I might be holding my hand against another button, or the menu is being displayed in the viewfinder because something triggered the eye sensor, but I am keeping an eye on this. I do believe their are a few firmware glitches that still need to be ironed out.
I have spent many years as an IT user and manager and this event is important. What helps solve any technology issue is to identify common components or settings that would allow Nikon to reproduce the issue. It appears you are in contact with the individuals who also had Z9 issues, so take some time to discuss what might have been common about this event and the way you were shooting. Consider not only camera settings, but also all lenses, FTZ adapters, and any other equipment used in conjunction with the Z9s. Usage habits are also important, were lenses changed, was the camera powered off and back on from time to time or was it left powered on for hours, were you shooting video or just stills, were cards changed, and many other things could be helpful. Were your Z9s connected to SnapBridge, NX Field, or any other remote trigger or software. Doing a little brain storming could help identify what caused the issue.
Since it appears that you all know each other, is it possible that the same lens was on all failed Z9s? Was there borrowed or rented lenses? If 4 Z9s failed there just has to be common link that contributed to it.
I do think there might be some sort of environmental factor as well since you were all shooting at the same event, but I have no idea what that could possible be.
Keep us posted and what you come up with and what Nikon finds.