Is it safe to lock bootloader?

[yvmuell]

Yes the Fairphone tutorial is not good or complete.

Those information is spread over the forum in various topics

Can't flash stock FP4 ROM

Definitely check fastboot flashing get_unlock_ability immediately before locking. And don’t boot into Android after flashing it, the first boot resets it to 0 (The reason being that’s how you get your phone delivered and there are apps that check for that). Flash → Check → Lock

I personally would also not lock my bootloader.