I've had 4 sets of hope brakes (m4, mono m4, mini, mono mini) and the only problem i had was a sticky piston in one of the minis (the original, silver caliper), which was easily fixed by my LBS.
When you say "M4", do you mean the original M4 (silver caliper), or the newer Mono M4 (black one peice caliper)?
I reckon that the original M4s were the most powerful, and should be plenty enough power to stop you, no matter what you are doing. I could never get rid of the noise though (and heaps of others had the same problem), and it drove me crazy.
The mono m4 is much quieter, are light for 4 piston brakes, and have probably a bit less power than the m4 (185 vs 180mm rotors), but a 180mm rotor up front should be fine - for either a medium/heavy guy who is trail riding, or a light guy for DH. Go the 200mm rotor if you are into DH and aren't light. I use mine for trail riding, am a middleweight (nearly 80kg), and am very happy with a 180mm mono m4 up front. It doesn't have the inital grab/bite of some other brakes - which is good and bad. The good thing is that the power is dailed on in a nice progressive way (is that called modulation?), instead of you getting pitched over the bars by some other older/cheaper brakes. The bad thing is that, now i have some formula oro's which seem to have the best of both worlds - a nice initial grab to them, but then the power dails up progressively from there. With the oros, it seems that I can dail up the power with less lever pressure than the mono m4, even with a smaller front rotor on the oro. That said, even though the mono m4 might need a little more lever pressure to get some power, I am getting plenty enough power from my mono m4s for what kind of riding I do, and quite often find my rear tyre touching down without even knowing it was off the ground in the first place - smooth brakes.
If you prefer grabbier/bitey brakes, which a lot of people do, don't get the hopes. It seems that a lot of people are used to those kind of brakes, and find the hopes lacking when they try them (ie. cause they don't get the same braking power for a given lever pressure, they conclude that hopes aren't powerful).
I hope this helps.