Today I will talk about using Raspberry Pi 4B as a Ubuntu 20.10 desktop, on a 4K TV.
If you are interested in using Ubuntu 20.10 on a Raspberry Pi 4B with 8GB of RAM and want to use a TV as a display I have a bit of advice that can help you out and save your time.
1) Do not upgrade from 20.04 - after upgrading the essential boot section changes won't happen, and you won't have hardware acceleration. Unless you know what to change, just install 20.10 from scratch.
2) You need to edit
/boot/firmware/config.txt and add
hdmi_enable_4kp60=1, preferably to the
[pi4] section. This setting is applied on boot. Your Raspberry Pi should have proper cooling.
3) You need to use the micro-HDMI port next to the USB-C power connector.
4) Your TV needs to be set to "PC" mode. Details differ but at least with my TV I was only getting 30Hz in any other mode. It may be related to the color format, I don't know.
5) If you boot and see the login screen just fine but get a blank / no signal screen after logging in then look at your
~/.config/monitors.xml file. I had the 30Hz refresh rate selected there and (again) it was not accepted by my TV in PC mode.