A Ring Cheek piece introduces poll pressure as well creating leverage on the mouthpiece. The 4 Ring has 2 rings on the bottom to adjust the amount of poll pressure and leverage you want.
The Mullen distributes pressure evenly over all the structures of the mouth, while this avoids pressure points, it does make the Mullen a fairly strong bit.
The material used allows the bit to remain light and it encourages salivation. It is particularly well suited to horses that object to metal mouth pieces.