This may be a stupid question but I'm just wondering why we don't have 4" shotguns shells. If you're facing a big bear that extra power would be helpful, or am I missing something here?
Good question.
The simple answer is that there are precious few 12 ga loads that can't fit into a 2 3/4" shell. Let alone a 3". Let alone a 3 1/2".
In the early days of cartridge shotguns, 2" shells were common. Then, cartridge vendors developed "long range" loads and such and 2 1/2" became common. But, it should be pointed out that this was in the days of black powder, when more powder was the solution for achieving more power. Next came 2 3/4" shells.
But this was around the time nitro powders were commercialized. Nitro powder is so hot it needs to be dumbed down. Various "filler" powders are mixed with nitro granules, both in a variety of granule sizes and ratios, to produce powder blends meeting 'volume of gas creation' and 'speed of burning' specifications. The significant difference, though, is that nitro powders occupy less volume. This leaves more room for shot. Plenty of room, in fact. So the world continued to turn without complications.
However, there has always been, and there continues to be in irrational circles, the notion that more shot is better (ie. If 7/8 of an ounce is good, then 1 ounce is better, etc). The same theory holds that a given measure of powder need be increased to provide more speed. These kind of beliefs influence marketing types to create longer shells and use names like MAGNUM. There was plenty of this kind of marketing nonsense going on already, but the evolution of wetlands management and mandate for non-toxic shot made some of this less ridiculous. Steel shot is noticeably lighter than lead and so requires larger shot sizes which, in turn, take up more space in the cartridge.
Still, 3" should be adequate for most everything except maybe specialty military rounds. Having said that, sometimes we have to buy what the manufacturers are making.