The 100v is normal for all DK machines, but yes your 4-board stack is an early version compared to the preferred (just for troubleshooting simplicity) 2-board set.
You seem to have a connectivity indicator that just lights up with power. I'd start by using a multimeter on the power supply connections (the silver box on the plywood mount). Here is what I pulled from my connectors recently:
4yellow +12v (multimeter read 12.0/11.9)
5green +12v (multimeter read 12.0/11.9
6blue +5v (multimeter read 5.1)
7purple +5v (multimeter read 5.1)
8gray -5v (multimeter read -5.1)
9white +24v (multimeter read 23.6)
1brown -5v (multimeter read -5.1)
2red -5v (multimeter read -5.1)
3orange -5v (multimeter read -5.1)
4yellow -5v (multimeter read -5.1)
9white +5v (multimeter read 5.1)
10black +5v (multimeter read 5.1)
I have the 2-board set, so mine only requires these two connections, although the other connector is present. If I recall correctly, the third connector links to the SOUND board in the 4-board stack.
Once you've confirmed correct output at the power supply, you basically need to confirm that that power is connected to and maintained to your PCB stack, and to your monitor chassis.
If the monitor has power and the PCB does not, you'll get a white screen.
If the PCB has power and the monitor does not, you'll get a black screen.
To me it sounds like either the wiring to the monitor is not connected (simple fix) or based on what you've said historically, the more likely situation is that the monitor chassis has a power issue (harder fix).
Confirm power connections to your monitor chassis, maybe snap some more pics of the back of the monitor, and report back.
EDIT for a safety tip, although I imagine you're no dummy. The monitor will hold a potentially fatal charge long after the game has been unplugged. Don't go touching the monitor yoke or unscrewing anything on the monitor chassis before discharging the monitor safely. If you don't know what I mean by this, don't attempt the repair yourself.