The iodéOS flashing instructions for FP4 were easy to read for a techie, yet alternative Android OSes newbie like me. Yet they lacked enough precision that I may have shifted to another alternative OS for my phone, were I not off duty these days. Please allow me to show you how and even make a couple suggestions.
Steps 1-3 were fine by me: just follow the linked and well done Fairphone instructions.
fastboot flashing unlock
This had my FP4 reboot to FP’s stock Android, loosing the USB debugging setting I just made. Since it was the same OS / environment, it took me some time to realize.
- is fine by me.
press POWER+VOLUME UP until reboot in recovery# didn’t work here. I had to:
press VOLUME DOWN (or UP) until I see « Recovery mode » on the top,
then press POWER button.
- is fine by me.
- From recovery => Apply update => Apply from ADB => adb sideload <iodéOS for FP4>
This point happily mixes on-screen instructions with command to be typed on the computer. Which might be a bliss for people used to flash their phone, but how are others supposed to know? The following instruction would have make sense to an Android OSes newbie like me:
On your phone, from recovery » Factory reset, press UP until the top left arrow is selected then press POWER
From recovery => Apply update => Apply from ADB => ADB Sideload . Then from your computer type:
adb sideload <iodéOS for FP4.zip>
$ adb sideload iode-3.4-20221215-FP4.zip serving: 'iode-3.4-20221215-FP4.zip' (~80%)
adb says something like this when it’s done:
Total xfer: 2.00x
 (optional) You can check how you did so far by looking towards the bottom of your phone’s screen:
-- Wiping data... Formatting /data... Formatting /metadata... Data wipe complete. Now send the package you want to apply with 'adb sideload <filename>'...