The 4CM is from Yamaha's Custom Series.
It has the same facing length as the Standard Series mouthpieces (23mm), but a tip opening of 1.7mm (same as 5C).
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.