I like rigid MTB.
Shimano Deore XT BR-M755 Quad Piston Disc Brake Caliper !!!
Shimano Deore XT BR-M755 Quad Piston Disc Brake Caliper
What is the advantages of quad piston design when comparing to the regular type XT ?
Depends on which XT version you're comparing it to. Compared to the 765 and 775 series it has a lot more power and better modulation along with better fade resistance. Against the 785 series it's about even or maybe down a bit in power, hard to say since the rotor sizes weren't the same on the bikes I rode. I've found modulation & control is better with the 755, the servowave levers on the latest XTs have a bit of a wonky spot in the transition that I can't quite get used to, I sometimes find it a bit harder to keep the brakes from locking up. As for fade resistance I don't know, I haven't had a chance to take a bike with 785 XTs on a sufficiently big downhill.