Thank you for your effort, and hints/help.
I had only written the initial post in the hope of a miracle, that there would be another way out than asking fairphone support for an EDL to clean the bootloader.;--())

@MrT10001 said in fp4: another brick in the wall:

... If Law enforcement can do it and do it on locked, encrypted iPhones (they are difficult to get into iOS unless you have expensive tools), expect the criminal fraternity to have to ability to do it.

I don't have $2k to buy an EDL device, and I certainly don't belong to a criminal organisation, ...damn, now I could use something like that
...;-)lol

No I don't have a FP (yet) as they are expensive here in comparison to other devices and from what I have seen

agree! But it was one of the most comparable thing I found for linux- and lineage-derivates without PWM.

and read have issues that involve sending them back to FP to recover - read here.

I should have read this things before!;-)