As long as its not possible for yourself to reinstall FPOS this will be done in the repair center or by Murena, when the process goes through Murena. We have no official statement, but we have some information, that as long as the FPOS files are not publicly available the FP4 can be reset to FPOS by FP repair center. However, I’m not sure if the OP wants this discussion here, but just made a decision to sell and buy a new phone.
Edit the links to the statements available for completeness