Ed- what sided ball would make a buckshot load in a 4 bore? That is, 3 or 4 layers of 3 balls that fit the bore in a triangle? There is probably a mathematical equation for figuring that out - it is beyond my match skills.
Just a gues is that .50 cal balls might work, ie: .595". Those would weigh about 182gr. X 9 = 1,638gr.

Sounds about right and what a deer/bear load - or for repelling pirates.