It is generally possible to relock the bootloader on FP4.
I suppose you’ll find all the details in the links mentioned above.
From what I’ve read and experienced I’d summarize it as follows:
As long as OEM unlock is enabled you’re safe and you can always unlock the bootloader as you’ve mentioned above.
It seems that the problem you’re facing occurs when flashing an OS with an older patch date over an OS with newer one. So maybe you came from a very up-to-date stock ROM. Secure boot (Android verified boot) doesn’t seem to like that. If this assumption holds true then you may be able to relock the bootloader after the next update (with a newer patch date). As said, always make sure OEM unlock is enabled before you try again.
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