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 Cherry Rollers roll in the horses mouth giving him something to play with and reducing the likelihood of the bit being held in the horse's teeth. They are constructed tightly so there is no chance of pinching. When pulling on the reins, the rollers allow the bit to move along the bars of the horse's jaws as opposed to rubbing.