Gold plated. Same cup diameter and throat size as the standard 4 (25.90mm diameter with 0.280" throat), but with less overall cup volume. The 4M (also known as the 4GM) is great for the tenor trombonist that wants the room of a larger mouthpiece withouta huge cup volume. A full natural sound is achieved in all registers with the balance found in the Griego Mouthpiece design. Sleek "Deco" style exterior is heavier than traditional models, allowing for a darker, more focused sound.