It means the alternator rotor (also ignition signal generator) sits 4 degrees further advanced, it's a stepped Woodruff key, I'm told this is a common tweak in karts. Anyway it gives you a dollop of bottom end, see the red and blue torque traces for a stock (and rather weedy) 1999 SV650:
http://www.johnnyrod.co.uk/dyno.gif