@fubuntu
To Clarify
FP4 comes with anti rollback protection and it will only trigger if your bootloader is LOCKED.
If your bootloader is UNLOCKED, then anti rollback protection is ignored

From what you read in the /e/OS installation instructions

Caution: The FP4 comes with an anti-rollback feature. (...) If you try installing a version of /e/OS based on a security patch that is older than the one on your device, you will brick your device.

That is true if you try to relock your bootloader, as in the detail of /e/OS installation instructions caution section

• Rollback protection errors trigger if you install an update whose version number is LESS than the rollback index’s value stored on device.
• (...)
• Rollback protection errors are FATAL when the bootloader is LOCKED.
• Rollback protection errors are IGNORED when the bootloader is UNLOCKED.

In short, if you unlock your bootloader and downgrad your FP4, you won’t trigger anti-rollback protection as long as you don’t relock the bootloader.

Also, you cannot relock your bootloader with Ubuntu Touch installed anyway.

@Keneda It's safe to downgrae FP4 to FPOS11 as long as you don't relock your bootloader. My FP4 came with FPOS13 from the factory, and I successfully downgraded to FPOS11 and install Ubuntu Touch in April 2024. Just DON'T relock your bootloader, as the FP4 will reset get_unlock_ability to 0 after the first FPOS11 boot. If you accidentally relock your bootloader with get_unlock_ability: 0, then there is no way to unlock your bootloader again.

If you want to retain get_unlock_ability: 1 after the downgrade, just don't boot FPOS afterwards. (Source: https://forum.fairphone.com/t/how-do-i-obtain-get-unlock-ability-set-to-1-on-an-unlocked-fairphone-4/105018/4)

As of 4/25, you can now try Lomiri with Ubuntu 24.04 LTS on Ubuntu Unity
They have testing iso for Ubuntu Lomiri 24.04 LTS and comes with installer.

I'm now switch my Surface Go 3 tablet from Debian Sid to Ubuntu Lomiri 24.04 LTS

@Keneda
It should be the same as described in the /e/OS installation instructions for Fairphone FP5 (Especially in the detail of caution section) and the Manage the Bootloader guide. However, I would suggest asking others who have successfully downgraded their FP5 or conducting further research first, as only the FP4 is available in my country (Sadly the sales representative in my country does not introduce FP5), and I cannot test with the FP5.

Regarding get_unlock_ability, I can’t confirm if the FP5 also has this issue where get_unlock_ability resets to 0. However, I would recommend checking this after the downgrade.