Thanks to the good folk who put the hard work into building a UEFI implementation for the Raspberry Pi 4 which "just works", allowing you to install Debian straightforwardly, and especially to Pete Batard who has written up the process and collected a zip file together.
Not quite so straightforward ...
I have an early model Raspberry Pi 4. I wanted to install Debian on an SSD connected via a cable to a USB3 port. It turned out that the version of the software in the EEPROM would not boot reliably so the first task was to update this with the latest stable EEPROM available from the Raspberry Pi downloads.
The easiest way to do this was to boot an SD card with Raspbian on. Once that was done, I had a Pi that would boot from an SSD.
Untar the files
A tarball of UEFI from Pete's Github repository at https://github.com/pftf/RPi4/releases - latest is v 1.29 as at 20210814.
Plugging in the SSD to another machine to format the drive: msdos format, one ESP partition in FAT32 and marked bootable and the rest of the drive blank.
One aarch64 DVD image from the usual place.
Untar the UEFI tarball into the ESP partition you've just made
Plug the SSD into a USB3 port on the RPi using a USB -> SATA cable
Write the aarch image to a USB stick using dd and place that into one of the other USB ports. Add a keyboard.
Power up the RPi4, hit Esc and work your way through UEFI to select a boot device and go, save the settings and go.
The install is almost identical to any Debian d-i install.
There is a setting in UEFI to reclaim the 1G of memory that was masked out, there's a setting for control of the fan shim if you have that style of fan.
End result - happiness
Done the other day and sitting next to me on the desktop.