Thanks for the confirmation starkruzr!
I just finished up the root process and it all worked eventually, but there were some caveats that I figure might be helpful to list here for future readers who stumble across this post.
First of all, the OTA update that was distributed to my device wouldn't work. I think it's because it was an incremental update (it was only around 400 MB). I tried unrooting my device first, but that didn't work either. I think because we directly write the patched boot image with edl now instead of using the fastboot approach, magisk doesn't keep a backup of the unrooted boot partition, so the update would detect the rooted boot image, silently fail and hang until I rebooted the device.
To resolve the above, I followed
this guide to get the full-size encrypted update file. The adb steps aren't really necessary, however. I just used my pc's file explorer to put the update.upx file onto the root directory of my boox's internal storage, and when I went into the firmware update page, it picked up the file and updated fine from there.
The rest of the root process was standard, except with the edl caveats. My notes for the edl steps are
here. Similarly to
vedri in this reply, however, I had to do all the operations on the boot_b partition and not boot_a. Boot_a has worked for me previously, so I imagine it just switches which one is active between each update.
As a final caveat, it turns out you don't need to specify the loader file when using bkerler's edl utility, at least on Linux (unless it cached the one I used from previous invocations of the commands).
I hope this can be helpful to others trying to root v4.0!