The MB 04 is similar to the MB 02 in that it will rotate on the tongue and apply tongue pressure. However, it has a small (¼”) port that introduces some tongue relief to the horse. Acting as a release or reward, the port allows the horse to swallow and move his tongue when he is moving forward and relaxed. The mouthpiece is curved, and it wraps around the bars and lips instead of pinching. It is available in four standard sizes, which are 4.75″, 5″, 5.25″ and 5.5″.