Using this guide from XDA combined with this XDA forum post I was able to extract hidden FP4 fastboot commands from the bootloader. They are the same on FP OS and /e/ OS.
/e/ OS extraction on Linux
unzip IMG-e-0.21-r-20220112156786-stable-FP4.zip
binwalk -e abl.img
cd _abl.img.extracted
strings -f 3078 | grep oem
3078: oem clear-rollback-index
3078: oem enable-charger-screen
3078: oem disable-charger-screen
3078: oem off-mode-charge
3078: oem select-display-panel
3078: oem device-info
3078: oem enable-root
3078: oem disable-root
3078: Enter fastboot oem off-mode-charge 0/1
Unfortunately, these commands won’t help us.
What happens if you try to boot the newest boot.img from code.fairphone.com or the one included in the /e/ OS image?
fastboot boot boot.img