The 4CM is from Yamaha's Custom Series.
It has the same tip opening as the standard 4C, but with a facing length of 21mm rather than 19mm.
The Custom Series mouthpieces are characterized by a warm rich tone with exceptional clarity. Their accurate intonation and comfortable playability have made them popular with many leading artists.
Custom Series mouthpieces are made of a high-quality hard rubber which offers a deep resonance similar to that of natural wood. Initial shaping of the mouthpiece ‘blank’ through to the final hand crafting of the facing is done by Yamaha’s experienced artisans for complete control ensuring consistent high quality.