Wow Bill, You just educated me. I had no idea Bill Moodie made 4 bores.
But bill is a superb maker and he understands barrel making science as well or better then most other makers. I would assume you have one that is "Forsyth rifled" so I would say to use a ball that's about .0202 to .025 under bore with a .010 to .012 patch. Some of the very large guns can benefit from a felt wad too. But I would just use a patched ball to start out.
Who is making the rifle for you?
Steve