Just for the record: I have also exactly the same problem with my few days old FP4. So it affects a pretty high number of people. The device is stuck after flashing back to stock OS and relocking the bootloader. All attempts to reopen the bootloader via fastboot commands fail. I read a bunch of support threads and I think the device is simply bricked. It should be covered by the warranty since every step followed the official guides.
I opened a ticket at the customer support on 2022-03-17 and will report when there is an answer from their end.