Hello!
I am following your thread here and I guess the main issue here is not “trapped in fastboot” nor “corrupted custom OS” but just that the PIN (of the original owner) is not known and we need it to unlock the FP4.
So basically this situation could happen to anyone who has forgotten their PIN… 
As already said, triggering a factory reset is risky as it could trigger FRP if the original owner has set up a Google account.
My best guest would be to ask your seller if he still has the contact of the original seller, and ask him for help to unlock it… 
Or you could also try the most common PIN (like 0000,1111,1234) - assuming that the phone is locked by PIN. Whenever you plug a USB-C-dock with a USB-Mouse [EDIT: or replace the defective screen module] you can check if it’s PIN, gesture or fingerprint locked… 