The loader for the FP3 wasn’t signed, the one for the FP4 is.
A generic loader for the chipset doesn’t work with the FP4, which is why there’s no working loader so far.
Look, I’m all for getting access to the loader, I’ve argued as much many times. It’s however also the case that the factory images for the FP4 are (or at least used to be, not sure about the current state) signed with testkeys, so a loader getting leaked is really not in the best interest of people running regular FPOS.
I agree, there always needs to be a way to fix devices. If the manufacturer doesn’t want to repair it without an invoice, then they should be required to provide independent third parties (who might be bound by NDAs in this case) with the tools to do so.