4 digit or 6-digit PIN makes no sense on OS X, but is that actually on iOS 7, and not OS X ?
This thread may help (or not):
https://forums.macrumors.com/threads/1438822/
Or, a trip to a "genius" - you should bring proof that you own your Mac when you go.
Would it be possible to rather boot in Target Disk Mode?
Nope... this has the same effect as an EFI password that prevents the boot source from changing.
The process in the thread DeltaMac linked can work on pre-2011 models where the EFI lock could be cleared by removing a stick of RAM, but if OP has a 2011 or newer machine a visit to the Apple Store (along with proof of ownership) is going to be required.
OP's machine has been locked down with a remote Find my Mac command.