I’m really sorry this has happened to you @efesto85… I’ve not had my FP4 for long, but to be clear the problem you’ve faced upon relocking the bootloader after flashing the stock FPOS through fastboot is a common issue that Fairphone is responsible for (even if it is not part of their services under warranty).
Here is the message #175 on another thread where a number of users have faced similar issues, bricking their phones and getting it repaired by Fairphone either for 30e (if considered out of warranty) or for free by @pjaap:
Update on my side:
After I contacted Fairphone support the device was send to Cordon for a repair.
This company replied immediately after getting the device that repair cost is 649€ (lol)
After a few more weeks (Fairphone support was not answering) I called Fairphone and they intervened at Cordon and one day later the device was fixed for free and now I have it back.So please don’t get trolled by Cordon and call Fairphone directly. This is much faster than my text mail. I had a very nice chat on the phone
So definitely do not give up and reach out to Fairphone directly, because Cordon seems to be the problem!