I have to disgree to a point. If you consider how the British proofed their guns, a 4 bore is considered anything over ,903 and under 1.026. The 4 would be proofed as a 5/2 for a very large gun. Refer to Greeners "The Gun". Not saying technically you are wrong but from how the British proof house marks them it would be incorrect. The 6 is considered a 4.