Bach 4 vs Bach 6½A (small shank)
The Bach 4 has a wider cup diameter, the Bach 6½A (small shank) features a deeper cup.
Cup Diameter
The Bach 4 has a cup diameter of 26.00 mm / 1.0236 in compared to 25.40 mm / 1.0000 in on the Bach 6½A (small shank) — a difference of 0.60 mm / 0.0236 in. A wider cup generally produces a fuller, darker tone but requires more air support.
Cup Depth
The Bach 6½A (small shank) has a deep cup while the Bach 4 has a medium cup. Deeper cups favor a darker, richer sound; shallower cups provide more brightness and easier upper register.
Throat Diameter
Throat diameters are essentially identical at 5.85 mm / 0.2303 in, so resistance feel will be comparable.
Which is better?
Neither mouthpiece clearly dominates the other — the right choice depends on your embouchure, playing style, and the sound you're after. Try both if possible.