4 and 8 gauge shotguns have been banned from hunting in the US since 1918. If I recall correctly, a 4 bore standard load of shot was 2 1/2 oz, not that much more than some newer 10 gauge loads.
4 bores are still used in the UK for pass shooting waterfowl. One or two BB members like MacFarlaine can contribute more about it.
As to this particular 4 bore, I do not want to try it.