Your instructions at https://github.com/pimoroni/hyperpixel4 work great as does the hardware (aside from the touch/mouse configuration as detailed at https://github.com/pimoroni/hyperpixel4/issues/22).
However, how do I temporarily revert to using a full-sized screen over the HDMI cable? I obviously don’t know Raspbian/Linux.
OK, I figured it out - but it SHOULD be part of the documentation.
In Raspberry Pi, open the command window, cross your fingers, and enter:
sudo ./Hyperpixel4/uninstall.sh
Then reboot.
If your going to switch back and forth editing the config.txt file may be a better option. hdmi_force_mode is the one I “think” you need to use.
https://www.raspberrypi.org/documentation/configuration/config-txt/video.md
To turn it back off and go back to the Hyperpixel just put # in front of the lines you add. A # at the beginning of a line makes it a remark and its ignored. Take them back out and that code / command is acted on.