" If all four engines were to fail like on BA009, the only hydraulic system remaining is No.4, ..."
Hi, Rich.
I'm not sure why you'd think that the #4 system would still be working and not the others? Windmilling will provide adequate hydraulic power to control surfaces (and, as I understand it, down to speeds a lot less than 250kts).
Depending on the hydraulic system (1,2,3,4) in question, hydraulic pumps can be powered by engines turning (mechanical input from the HP engine rotor), electric or pneumatic power. Main AC Bus electrics fail at around 50% N2 (or N3)...i.e just below idle... and, most likely, unless you were in a power dive, I doubt that the engines would spin fast enought to provide AC power to power the Electric Hydraulic pumps (Elec Hydraulic Pumps cannot be powered by Standby AC Elec power.... The batteries would go flat in seconds). Pneumatic-powered hydraulic pumps also need the engines to be turning, but the aircraft needs main bus elec power to control certain elements of the bleed system (I'm not 100% sure how the bleed system would behave without main AC power (certain valves...e.g. isolation valves... need certain Main AC busses to operate)).
Bleed and electric power can also be provided by the APU. However, I believe most, if not all, 747-200 airlines have modified the APU so that it can't be started in the air. I don't believe that any 747-400 has this air start facility (I did hear that one person figured out a way to start the APU by pulling certain circuit breakers in flight, but I never followed it up). If all engines have failed due to volcanic ash, I don't think the APU would fare much better. There is also a fire risk (only one extinguishing system on the APU) and the drain on the APU battery to consider, which, on the 747-400, powers flight instruments and a few other vital items. Several failed starts may leave you with a seriously depleted battery.
"....however this services the right inboard and right outboard aileron. The left inboard and outboard ailerons are serviced by hydraulic systems No. 1, 2 and 3."
If the #4 Hydraulic system was the only one available, the #5 spoiler on the left wing should still be available for lateral control, but I can't tell you how much turning effect this would give you. Also, don't forget that ailerons move up AND down. Up will lower the right wing, down will lower the right wing. Not quite as effective as ailerons on both wings for turning, but...
Another factor to consider is the effect the loss of AC power has on the Air-Ground system. Will certain flight controls behave differently if the Air-Ground System powers down? Speedbrake panel deflection is limited in flight.. If the aircraft Air-Ground system switched to ground mode onpower down, you might get a little more speedbrake deflection than you bargained for (if for some reason you needed to use the speedbrakes): Here's a question for the 747 pilots... If there is an emergency landing field which is too close for a normal idle glide, do you use your speedbrakes... or try to maintain altitude for as long as possible and dog leg towards the landing field without using your speedbrakes?
Anyway, hope this helps.
Rgds.
Q.