Kelly 3C vs Kelly 4B-UK
The Kelly 3C has a wider cup diameter, the Kelly 4B-UK features a deeper cup, the Kelly 4B-UK has a larger throat opening.
Cup Diameter
The Kelly 3C has a cup diameter of 17.02 mm / 0.6701 in compared to 16.51 mm / 0.6500 in on the Kelly 4B-UK — a difference of 0.51 mm / 0.0201 in. A wider cup generally produces a fuller, darker tone but requires more air support.
Cup Depth
The Kelly 4B-UK has a — cup while the Kelly 3C has a very deep cup. Deeper cups favor a darker, richer sound; shallower cups provide more brightness and easier upper register.
Throat Diameter
The Kelly 4B-UK has a wider throat (4.37 mm / 0.1720 in vs 4.06 mm / 0.1598 in). A wider throat allows more air through, increasing volume and projection but reducing resistance.
Material
Both mouthpieces share the same listed material (Lexan polycarbonate).
Which is better?
The Kelly 3C is designed for general playing, while the Kelly 4B-UK targets orchestral use. Your best choice depends on your musical context.