Can't flash stock FP4 ROM

[Ingo]

I can’t really tell, but IIRC correctly one should always make sure the phone can be unlocked again in case things go sideways.

Trying to revert to Fairphone OS but get_unlock_ability: 0

Safe, no, I never recommend locking the bootloader, because there’s always a risk involved. You seem to be on the right version, but ultimately you’ll have to decide if you want to risk it. There’s really no need to lock the critical partitions, but in the spirit of following the official instructions, you can of course do it. If you do, check get_unlock_ability before you lock the bootloader completely.