I think this is the link to your motherboard... Or close enough. You're probably using Win7-64, since you have 8Gb of memory.
Nero claimed that "Nero 10" was their first level of good Win7 support, so this might be one issue, but I see a lot of Win7 users with Nero9-OEM being used. Just not with multiple burners. This might be one of those "good support" instead of "claims to support" issues.
What happens when you unplug 2 of the Burners and run 'on two'? Do those two work perfectly well for session after session?
I might be tempted to work this in steps - use Burner #1 & 2 for a day or two, then unplug those and see how #3 and 4 behave as 'the only two' - as if it's Burner Hardware Problem.
Then add in one of the Unplugged Ones as Unit #3 and see if the odd behavior follows. Eventually, this might also impact the power supply, which I haven't asked about. Flakey electricals will be impossible to diagnose from this distance - that's probably a "replace it and see if there's any behavior change" step.