Thank you, Nicolas and Krause.
I've managed to get Moodlebox running on my Pi 400 even before I read your (Krause's) reply which was confirming. I had initially copied the Moodlebox image to the downloads file of the Raspberry Pi 400 - that didn't do anything. After a night's sleep, I realized that I would need to copy the image onto the card outside of the Pi 400 OS, so I took the card out of the Pi 400 and put into my Windows laptop, and copied the image to the card while it was in my laptop. I took the card out and put it into the Pi 400 and the Pi 400 became a Moodle server which I've successfully accessed from my laptop.
Now, I will need to get another SD card and copy the Pi 400 OS onto it, so that I can switch them out as Krause suggests above. I don't really need another desktop computer, so I will wait a bit on that.
I'm more interested in discovering how I can actually acquire a Raspberry Pi 4. They seem to be sold out everywhere I look. I'm not planning on attending any in-person conferences anytime soon, so I could just hold up the Raspberry Pi 4 case that I ordered along with Raspberry Pi 400 when I'm doing a video conferencing call to explain the application I have in mind.. When, I ordered it, as I said above, I thought the keyboard was just a nice little keyboard that was included with the Pi 400 package from Adafruit, and thought that I would need to the case.
I will think over this process I went through and suggest some edits to the Installation Instructions on the Moodlebox website applicable to the Pi 400. It would be important, I think, to note that Pi 400 is useless as a keyboard/desktop unless you get another SD card. It is a very nice keyboard, though, and would be useful as another backup computer when I get the other card and figure out how to make the sound work with my monitor. My current monitor has a separate sound cable with a headphone type jack for sound (it does not have an HDMI port- I'm using adapters.) I assume that a different monitor with an HDMI port would enable sound for the Pi 400.
Since, I'm working only at SABIER http://sabier.org/ a very small underfunded nonprofit I founded, my contribution to the Moodlebox community will be limited to my experiences, for now. I'm hoping that my project will lead to increased demands for Moodlebox.