I think if you're careful in layout and design you should be able to use a standard sized lock. I've seen some of the later percussion pieces and while they are of course a bit chunkier through the breech I don't ever recall them using larger-sized locks. Obviously there's a trade-off and you'll have to juggle lock panel width, lock size, breech width etc. Also with modern barrel steels you may be able to get away with a slightly smaller breech than would have been typical 200 years ago, perhaps not dramatically smaller but just enough to help balance things out. Also given that a 4 bore is really intended for shooting something that could otherwise very quickly kill you or squish you or mangle you, I would want the fastest and most reliable lock on this planet! I think a Chambers late Ketland would work very well, as they are fast, reliable, and a good size; I have seen, handled and owned a number of similar English locks that are considerably smaller, so I actually think that this particular lock (on the average for the style) may be slightly on the larger size of average. I'd figure out how to make it work in an aesthetically pleasing way and then rest confident that the sucker was going to go off when I wanted it to do so, and quickly!