My experience with the FP4 is you can only lock the bootloader with a couple of “official” distro’s. And even then there are conditions where this can go bad.
I can use the default factory version provided by fairphone, I can use e/os.
While I can install and use postmarketos or a lineage image, I can’t lock the bootloader without turning it into a brick. I believe it is to do with boot partition signing by the CPU manufacturer and the android bootloader checking it.
Partially true, this applies to signed boot loaders, postmarketos can’t be locked like e/os or the default distro by FP