Could be, but I suspect that's probably a coincidence; if they had meant to use that system on a non-shotgun, the British convention would've been to call it "4-bore". Calibers are usually given with the number first too, but "4 calibre" would either mean four inches, or four times the bore diameter in length, depending on the context. My guess, though I acknowledge that it is only a guess, is that "calibre 4" just refers to the fourth one on whatever list it appeared on, and that there were other signal cartridge sizes (calibres 1 through 3 and whatever else) that didn't catch on. Similar in concept to the "Rifle № 1" numbering that was retroactively applied to the Mark III SMLE after World War I.

--G.
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