PCPLUS recently supplied a link to an online calculator which indicates the power supply needed for your particular computer :-
eXtreme Outer Vision - eXtreme tools for computer enthusiasts
Might be worth checking since you are running 4 hard drives.
The other thing to check is your drive cables.
I had a ribbon cable which failed due to an overheating graphics card (a wire was touching the centre of the fan, so no noise but slow rotation).
The insulation was softened by the heat and shorted out because the wires touched when I moved it - exit hard drive.