The alternators are geared to produce the correct frequency at the "high" (ie in flight) RPM (13820). There is also a "low speed ground idle" RPM setting of 9900-10300 that saves fuel and reduces noise. Number 4 engine has an auxiliary 2 speed gear box that allows the alternator to produce on frequency power at either LSGI or high and the bus is designed so that the two priority buses and the essential AC bus can powered with only No 4 engine running.
We always start the engine in low for two reasons
1) it is easier on the engine as you get lower start temps, and
2) There is a gearbox maximum pressure limit (175 psi) before you can upshift from LSGI to High. This is because with cold oil you can greatly exceed the max gearbox pressure if you were to go directly to high RPM on start.