I have deployed several X4200 on 3.9 (with mpi(4) backported from 4.0). AFAIK, X4200 == X4100. It just has some more PCI slots.
When booting, "RTC BIOS diagnostic error 2" is displayed, I'm not sure if that's relevant.
You might want to investigate that. Not sure, but I don't remember seeing that error on the X4200 boxes I had tested. BIOS update might be relevant. Perhaps it's also caused by bad hardware. I have not seen any stability problems with my X4200 deployments. They are not running as network firewalls, but as application level proxies, so the error you are seeing could be due to higher pps count. Unlike you, I didn't put anything non-stock in the box. 4 built-in NICs where enough for my purposes.
After the NMI, the system is at the ddb prompt, but the virtual console is unresponsive and I can't type anything at it. So far I haven't been able to get the serial console working, so I'm not sure if the unresponsiveness is due to the USB virtual console, or if the system is just plain hung up.
USB layer doesn't work in ddb> so you'll need the serial working to get useful debug data.
