Why only the outboard ("low speed") aileron drops when there is no hydraulic pressure on the 747-8?
The Boeing 747-8 (unlike the 777) is not a full fly-by-wire aircraft, but some fly-by-wire controls have been added to the original 747 design. The inboard ailerons are mechanically connected to the flight controls, while the outboard ailerons are not. This allows them to droop without hydraulic power.

From the Boeing 747-8 FCOM (9.20.6 Flight Controls - System Description):


  Roll Control
  
  Inboard ailerons are mechanically controlled and hydraulically powered. Outboard ailerons and all spoilers are electronically controlled and hydraulically powered.
  
  Aileron and Spoiler Roll Control
  
  Rotating either control wheel positions ailerons and spoilers. Above approximately 258 knots/.53 Mach, the outboard ailerons are commanded to the neutral position to prevent overcontrolling. The outboard ailerons become operational at lower airspeeds. Inboard aileron deflection is limited when flaps are extended beyond 10.
  
  To improve takeoff performance and reduce noise, the outboard aileron is deflected down for flap detent positions 10 and 20. The downward deflection is removed if the angle of attack is above stick shaker. The inboard aileron is deflected down when the flaps are selected to 10 or greater.
  
  All spoiler panels operate with the ailerons. Spoiler panels are deflected up or down based on Speedbrake lever and control wheel inputs.




The following image from modernairliners.com shows all new features of the 747-8 wing including the Aileron droop.