Yep, worked like a charm! 
Many thanks.
For others who may be interested, I…
- Updated the RPi4 eeprom as per this James Chambers page (The flashing of an update of the eeprom firmware requires a recent RaspberryOS image.) Depending on your RPi4 version, you may find that this step is unnecessary in your case.
- dd’d the 163 image to a µSD as before.
- Installed the µSD in a USB adapter, and slotted the adapter into one of the RPi4 USB sockets. (And made sure there was no µSD card in the RPi4 µSD slot.)
- Booted the RPi4.
- Watched IPFire boot and install without a hitch.
IPFire has subsequently booted from the USB port without problems. (It still doesn’t boot from the µSD slot.)
One caveat (of course): The flashing of an update of the eeprom firmware requires that the RPi4 be rebooted into RaspberryOS to complete the process. In my case, I had to move the µSD to the USB port in order for the reboot to work. The RPi4 booted RaspberryOS from the µSD slot after that.