Updated Mac to El Capitan is asking for 4 digit PIN
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.