According to the proof tables in Greener's The Gun and its Developement the 4 bore used 279 gr of powder and a 2791gr ball as a service load. This apparently a conical since an 4 guage ball should weigh 1750.
In Chapter 26 he notes a 4 bore using 12-14 drams (up to 385 gr) and a 1510 gr spherical bullet. Though this comes out to a 6 bore ball.
The proof tables give loads for bores up to 1 but I doubt that bores over 4 were used in shoulder fired weapons. 4 bores were about as heavy as a man could handle in field shooting situations.
I am pretty confident the 4 bore is the most powerful BP catridge load. Using ME as a guide in guns of this sort is misleading as to its actual effect on game.

Dan