Don't know about the B747-400, but I suspect it would be the same as the Classic on which placing the APU Master switch to ON powers the APU Fuel Shutoff to OPEN, initiates the APU Inlet door to OPEN as well as pressurize the APU fuel system. The APU Starter Circuit is deactivated until the APU Inlet Door is FULLY OPEN. That is the way Mr Boeing designed it to work.