OK, here's how to root the GL4. What I've done is taken Renate's post #37 from the XDA thread referenced in my previous post and fleshed it out a bit:
This presumes that you have ADB working and have already installed an alternate Android launcher on your GL4. If you haven't done any of this, check out
this post before proceeding.
Here we go. Note that our Nook can be a bit stubborn about responding to ADB commands, so if it doesn't work the first time, try again and/or ask for help!
1) Download the GL4 firmware to your desktop or laptop from
here.
2) Unzip the file.
3) Look for boot.img in the unzipped file and transfer it to your GL4.
4) Download the Magisk app from
here and install it on your GL4. (Note: that link is also a very helpful source of info for Magisk.)
5) Run the Magisk app.
6) You'll see two slightly shaded boxes, entitled Magisk and App, respectively. In the Magisk box, directly to the right of the word Magisk, tap Install.
7) Under "Select and patch a file", select the boot.img file you just copied to your GL4. If you don't see this option in the first Magisk window you arrive at after tapping "install", tap "next" and you will.
8) Once Magisk completes its work on your boot.img, it will say so and will tell you where the patched file is located.
9) Copy the patched file back to your desktop/laptop. You can use ADB pull if you wish.
10) Enter the command adb reboot bootloader on your desktop/laptop.
11) Then, this command: fastboot flash boot
the name of your magisk-patched boot.img. (I italicized that to indicate that you type the name of your Magisk-patched file instead of what's in italics!!) You can copy and paste the Magisk-patched file instead of typing out its full name. Tap Enter or Return, depending on what laptop/desktop you have.
12) Reboot to system. You can try adb reboot system or simply turn off your GL4 and turn it back on. You should be rooted now. Check by running the Magisk app once you are rebooted. Under the Magisk heading, besides "Installed", you should see a number. If you see "No", it didn't root properly. You can also install a root checker app to check root.
I hope I haven't missed anything!