To me, the perfect drilling would be a pair of 20 gauge barrels and a 7X57R rifle barrel with a quick detachable scope.

I agree, though, that your 4 barrel combo would be great for 99% of the world. For world wide use and pick up the last 1%, a pair of 12 gauge barrels over a .375 H&H Rimmed and a .22LR would take anything on the planet!!!