The twins need the inflight APU for their ETOPS ( Engines Turn Or People Swim ) third power source certification. The cases where 747's have had multiple engine failures have usually involved volcanic ash ingestion. They were all recovered following engine start at lower altitudes, perhaps not all four, but enough to appease the Gods of flight. The probabilities of losing 4 engines and not being able to recover at least one, are probably similar to the RAT not working when it's needed, so why go through the motions and carry the complexity. Even windmilling, the engine hydraulic pumps wil provide adequate pressure for the flight controls, and doesn't the bugga carry a honking great battery for emergency power. Don't know about starting the APU on the ground and leaving it running after departure - sounds odd. Some current whale driver will comment I'm sure.