There is a lot of confusing information being posted here.
It is impossible to windmill an engine fast enough to run the generator or to supply sufficient bleed air to run a hydraulic demand pump. ONLY the engine-driven pump will work when the engine is windmilling, and ONLY above approx 160 KIAS.
After the APU is shut down, it cannot (-400) or shall not (Classic) be restarted while airborne, so it is not an option for an airborne hydraulic failure. The APU generators shall not be used airborne.
While the Classic has 4 air-driven demand pumps (1 per engine/system) in addition to the engine-driven pumps, the -400 is available in several optional configurations where 1 or 2 of the air-driven demand pumps is replaced by an electrical pump. The air-driven pumps require bleed air from at least 1 running engine connected to the bleed-air manifold, and the electrical pumps will run (as selected/required) as long as there is at least 1 engine running and its generator is powering the synch bus.
The electrical Aux pump on system 4 runs ONLY on the ground, and cannot be run airborne (except in some cases of failed air/ground switch logic). It is sized to charge the parking brake accumulator ONLY, and will not reliably run ANY other hydraulic system.
I am aware of only 1 case of a 2-system hydraulic failure on a 747, and no cases of a complete, 4-system failure. While the 747-8 may have a RAT, I don't know why anyone thinks it necessary for hydraulic power (though it would be nice for emergency electrical power).