If any of you are as foolish as I am and purchased Samsung’s 49" CHG90 monitor over a year ago, here’s how to enable the full resolution.
The Librem 15v4 graphics card is an “Intel® HD Graphics 620” chipset. AFAIK, the Librem 15v4 has only one display port, which is the HDMI (v1.4) port on its right side. Due to the limitation of the HDMI port on the laptop, it appears to only support higher resolutions (up to 4096x2304) if the frequency is set at 24Hz, instead of the 60, 120, and 144 Hz offered by HDMIv2 and DP ports (both of which are not included in Librem 15v4, I think).
To enable 3840x1080 resolution on the CHG90, you will need to do the following in the terminal of your choice:
sudo cvt 3840 1080 24
which should output something similar to this:
Modeline “3840x1080_24.00” 126.25 3840 3944 4320 4800 1080 1083 1093 1098 -hsync +vsync
copy everything after "Modeline " and paste it after typing “sudo xrandr --newmode”, for example:
sudo xrandr --newmode “3840x1080_24.00” 126.25 3840 3944 4320 4800 1080 1083 1093 1098 -hsync +vsync
Normally with most other laptops, you would need to type “sudo xrandr” by itself to determine which display port your external monitor is connected to. With the Librem 15v4, it will more than likely be named “HDMI-1”, but type the xrandr command anyway to make sure; it’ll say “connected” next to it.
Now type (or copy and paste):
sudo xrandr --addmode HDMI-1 3840x1080_24.00
sudo xrandr --output HDMI-1 --mode 3840x1080_24.00
Your screens may flicker a little bit to enable the new mode, and voila, your frustratingly huge and low-res CHG90 monitor should work at full resolution. Problem is, it won’t be persistent, which I will now show you how to do. Just kidding, there’s already an Ubuntu wiki that explains how: https://wiki.ubuntu.com/X/Config/Resolution#Setting_xrandr_changes_persistently
I used 24 Hz and it’s possible that with just some more experimentation I might be able to up the Hz on my monitor, but I am busy finally being able to use the dang thing and so I will experiment later.