Updated Mac to El Capitan is asking for 4 digit PIN After updating my macOS 10.6.8 to El Capitan my Mac is asking for a 4 digit system PIN. I have no idea what that is.
The Mac has been locked via Find My iPhone. Whoever locked it has the code. Try logging into Find My iPhone on iCloud.com or on another Apple device and see if you can unlock it there.

If that doesn't work, somebody posted a (somewhat complex) work-around on the mac-rumors forums.

Last resort: go to Apple store with proof of purchase. They'll sort it out.