@kevin-lhotellier
You need a code to unlock Fairphone bootloader that only Fairphone can give you.
https://www.fairphone.com/en/bootloader-unlocking-code-for-fairphone/
And follow their process to unlock its bootloader (from step 2).
https://support.fairphone.com/hc/en-us/articles/10492476238865-Manage-the-Bootloader
Also be care that Fairphone 4 don't have the anti rollback protection like the FP5 has before downgrading it, or you could brick.
And if you succeed installing UT on it DON'T RELOCK THE BOOTLOADER, or it's a brick.