Your question is rather vague and confusing.
I very much doubt that you need to use 2 DVI inputs (which your question seems to imply), but it unclear why the monitor has 3 - it is probably designed to connect to 3 computers.
The Pi4 supports 4K ONLY through the left HDMI port. The 2nd port is for a second monitor.
I suggest you use a single connection, ...
I'm just not sure how to use the terminal to change the Desktop's active resolution without a reboot. I'm not sure if I should be looking for a way to change the framebuffer resolution, or X Server's resolution, or the Desktop Environment's resolution, or all of the above.
From my understanding, the Raspberry Pi GPU resolution is completely unrelated to the ...
I don't know what you mean by "HDMI hot-pluggable" (or indeed what the Foundation setting actually means by the parameter in config.txt).
I take your question to mean can you change the resolution post boot.
Normally the resolution (and other display settings) is set on boot and inherited by GUI.
It is theoretically possible to select a display mode with ...
Those are explained here: https://www.raspberrypi.org/documentation/configuration/config-txt/video.md
There is also this: https://www.raspberrypi.org/documentation/configuration/hdmi-config.md -- But if the group 1, mode 16 as above works, you don't have to worry about all that.
I've found a solution. I've gone to the raspi-config, disabled 4k support, re-enabled it, then disabled the forced resolution to default.
Now it is working fine. I hope this helps everyone with the same problem.
- Verify that the monitor cable is connected to HDMI0 (closest to USB-C power)
- If resolution has been modified from defaults, reset to default
- If available on the monitor make sure the default input is set to HDMI.
I think modern HP LED monitors have that setting available.
Having experimented with a virgin SD card install, I confirm Dougie's comment above: the default is HDMI, but if an HDMI monitor is absent, it falls back to using composite output.
I cannot yet explain why this doesn't seem to be the case for my OSMC installation. As such, the question is arguably in the wrong forum!