[Google Pixel 4 XL Questions & Answers] ihuntinde:
So.. for those that need it, here are my exact steps for a bootloader unlocked Pixel 4XL bought from the Google Store:

1. Download full factory image from here: https://developers.google.com/android/images#coral (10.0.0 (QQ1B.191205.012.A1, Dec 2019))

2. Extract the zip file, open the zip inside of that extracted folder and copy the boot.img to your phone's Downloads folder.

2a. I used a usb cable to my PC and put the phone in File transfer / Android Auto to do this)

3. In Magisk Manager, click Install. Click Install again in the popup. Choose Select and Patch a File.

4. Find the boot.img that you copied to your phone's Download folder. Select it.

5. Copy the magisk_patched.img file from your phone's Download folder to the unzipped factory image folder.

5a. Refer to 2a above. You may have to disconnect the usb cable and reconnect. For some reason, my PC wasn't refreshing the folder with the patched file until I did this.

6. With your phone connected and in File transfer mode. Open a command prompt in the folder you unzipped the factory image to.

6a. You may be able to shift and right-click then click Open Command Prompt window here. That will open the command prompt at that exact folder instead of having to navigate to it through the command prompt. If you are a newer version of Windows 10, you may only see the Powershell option. I am not sure if Powershell will work. However, to get to a "command prompt" to be able to run the flash-all and fastboot commands. Simply type cmd and hit <enter>. Then you can use the adb, flash-all, and fastboot commands as explained below. ** WARNING: I DID NOT USE POWERSHELL, USE AT YOUR OWN RISK **

6b. Type adb reboot bootloader.

7. Remembering this is a dirty flash (i.e, keeping all of your data) edit the flash-all.bat file in any type of editor (even notepad). Look for the line that says fastboot -w update image-coral-qq1b.191205.012.a1.zip and remove the -w. So, the line just above echo Press any key to exit... should look like this:

7a. fastboot update image-coral-qq1b.191205.012.a1.zip

8. Wait for the flashing to finish and for your phone to reboot and fully boot back up.

9. Once it's connected, put your phone in File Transfer usb mode again.

10. Open a command prompt and type adb reboot bootloader

10a. See 6a

11. Once the phone is in fastboot mode, type fastboot flash boot magisk_patched.img

12. Once that is done, type adb reboot.

You should now be on the December update and rooted.

If you get the error that says "should be flashed in fastbootd", I can confirm that you do not have a new enough version of fastboot.exe. You can do one of two things. First, download the latest platform tools from here: https://developer.android.com/studio/releases/platform-tools

1. Either hunt down every copy of your platform-tools folder (I had multiple for some dumb reason) and consolidate to just one, then update your platform-tolls. Then update your system environment variables and point it to this new folder (making sure it's the only folder pointing to a platform-tools version).

2. Download the new version and copy the fastboot.exe and adb.exe directly into the folder with the flash-all.bat. This will guarantee you are using the correct version.

Hope this helps someone...