I had the cluster running Einstein BRP4 work on the weekend because it was cooler. At the moment it consists of 12 nodes. I have 4 nodes in each of my Pi^4 cases and I have 3 cases.
I checked up on the running tasks using BOINCtasks and most of the tasks were nearing 90% completed after 10 hours, but 4 nodes seemed to be a lot slower as their tasks were only 40% done. Then I looked at the Pi^4 case that the 4 nodes were in and noticed that the red power LED on all four were off, but they were still running and doing regular disk access. The red LED is normally on constantly. If its blinking or off then the Pi thinks there is a problem with the power supply. If all 4 of them are off and they are plugged into the same USB charger then its probably the USB charger.
Fortunately I had a spare 5 port charger and swapped it out which got them going again and proved that it was indeed the charger. Even though the blue LED on the charger was on, indicating it was working, it obviously wasn't. I don't blame the charger really - Its not rated to run 4 Raspberry Pi3B+ (which in theory can pull between 8 and 10 amps) and a couple of 60mm fans constantly. The tasks have since validated despite this.
