To anyone that might be having a similar problem.
Problem: Raspberry Pi 4B 4Gb daily lockup, no logs, no video output.
I build a 6 node Raspberry PI 4 Kubernetes cluster (mix of 4&8Gb models).
All nodes are loaded with ubuntu 20.04 running k3s.
Node 6 would lock up once a day requiring a power cycle to get it up.
Core Electronics suggested a firmware update, that seemed to help, but the lockups continued.
Just per chance I noticed that the LEDs on the network card would go out when locked up.
I changed the network cable (reused exact same port in switch) and now about a month later, still no lockups. Not sure if it is a combination of the firmware update and the network cable change.
But if you have an unruly Pi locking up, try switching the network cable.