How to toggle between 2 specific resolutions via keyboard shortcut?

This answer was more helpful than I originally realized.

1. In Ubuntu 20.04, go to Settings → Displays.
2. In the Resolution dropdown, count the position of the resolution options that you care about. (In the screenshot below, "3840 x 2160" is 1st, and "1920 x 1080" is 7th.)
3. In Terminal, xrandr -s 0 -r 30.00 will set the system to use the 1st resolution at 30 Hz (or 29.98 Hz actually, somehow), and xrandr -s 6 -r 30.00 will switch to the 7th. Test those commands.
4. vim toggle_resolution.sh and paste the sample bash script shown below (and edit as necessary).
5. chmod +x toggle_resolution.sh
6. Run ./toggle_resolution.sh a couple times to test the toggling.
7. You could create a hotkey in Settings → Keyboard Shortcuts to run that bash script file.

Sample bash script:

#!/bin/bash

# https://askubuntu.com/a/1351112/48214
X=$(xrandr --current | grep '*' | uniq | awk '{print $1}' | cut -d 'x' -f1)
# echo "$X"

if [ $X == 3840 ]
then
        xrandr -s 6 -r 30.00
else
        xrandr -s 0 -r 30.00
fi