Having the same trouble with my FP4 yesterday, i wondered that there were no statement in this old thread if the problem still exists.
So i just want to inform everybody, that after i have updated my FP4 to actual iode-6.12-20260131-FP4 i could boot the system after i executed fastboot flashing lock_critical but after fastboot flashing lock i got the message Your device is corrupt. It can’t be trusted and will not boot.
So i first executed fastboot flashing unlock and then successfully followed vince31fr‘s recommendation (from 03/2022 - see above).
After that my FP4 has now actual iodeOS with re-locked bootloader and boots perfectly with AVB re-enabled and finally also with re-enabled OEM lock.
As i missed a more verbose / detailed description while re-enabling the bootloader i have logged the commands and their output for others in the same situation.
To keep this thread clean i link my session log to my own webspace and hope this is ok with the forum, You may view or download it from here.
Have fun flashing and using iodeOS on your FP4.