The Halifax marked guns seem to have a different numbering sequence then the graded monobloc guns referenced in the chart, above. The biggest problem with the chart above is that it was specific to guns sold in France. Exports were numbered differently.
The capital letter B on the guns barrel lug seems to indicate a gun with brazed, non monobloc barrels (tip of the hat to Dustin on that one). The most important clue on this gun is the “No 4” you see on the flats.
The Halifax existed because there were economic times that didn’t allow for people to part with the money for a graded Darne, which, was always an expensive gun. When times were really tough, there were several grades of Halifax gun. Your No 4 was produced in a window from about 1930 to sometime in the late 1930s. A number 4 was the most expensive of the Halifax guns.
A 16 gauge Darne is a pretty sweet spot to be in for a gun you can comfortably carry all day without growing weary of same. It isn’t a high volume clays gun, but, you already knew that. If you search the forum you should be able to come up with tips I posted here about a million times on how to keep your gun healthy, and you happy with using it, otherwise, ask again.

Good hunting. That is what it is for.

Best,
Ted