True" 4 gauge is 1.052", 5 ga is .976" & 6ga is .919". Most 4 gauge guns which have proof marks will be proofed as a 6 though they are chamber marked as 4 gauge.
I have been told this was because the 4 was first introduced using thin brass huls. When paper hulls began to be used the chamber size was retained but the bore size dropped to be compatible with the smaller wads required in the thicker walled paper hulls.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra