Kelly 4B-UK vs Kelly 6V
The Kelly 4B-UK has a wider cup diameter, the Kelly 6V features a deeper cup, the Kelly 4B-UK has a larger throat opening.
Cup Diameter
The Kelly 4B-UK has a cup diameter of 16.51 mm / 0.6500 in compared to 16.00 mm / 0.6299 in on the Kelly 6V — 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 6V has a — cup while the Kelly 4B-UK has a — 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 4B-UK is designed for orchestral playing, while the Kelly 6V targets dark/deep use. Your best choice depends on your musical context.