Slang with engineers and drafters for a number with four zeros in a row which is typically found in a top level assmembly drawing. Originated with the aerospace industry back in the 50s. In nerdspeak it has morphed into meaning anything that describes the big picture or the master plan.
"Bob, have you finished the four balls for the flux capacitor installation yet? The PHB is down my throat and threatening to take my stapler if it isn't done by this weekend."
"Dude, what's the four ball on backpacking to the hotsprings this weekend? Your sister coming?"