Just because it's 4 piston doesn't automatically make it a better brake...
hydraulic fuel line pressure, disc size, pad compound, pad backplate, disc type and disc pattern and proper pad setting (getting hot so they flattern themselfs against the disc)
these all equal up to a good brake, a hope m4 on a naff disc will feel crap so you could say it the brakes but infact be the disc
Yea right, that was comment was too simple. Four pistons equal a larger brake caliper and more braking fluid which in turn has greater heat capacity which in turn makes the brake much more stable during a run. I´m 68 kg so I never had the initial braking power problem, but on steep and long tracks like those in Morzine I got massive fading at times from smaller brakes.
Dunno really about those Shimanos with their fancy cooling system, friends say it works.