I installed Ubuntu 20.04 with Xfce4 DE (`xubuntu-desktop`) on a Raspberry Pi 4 4GB.

System runs good but randomly it shutdowns/halts, I don't known exactly what happens because I use it as headless server and connect via SSH or VNC.
When I can't connect to it, I go where the Pi is and I find it with both leds (activity and power) off. There is a little fan connected to +3.3 V pin of GPIO for cooling and it is on...

I'm sure there aren't hardware failure because the Pi runs like a charm with Raspbian Buster OS!