A 4 bore (bore is the same as gage in this case, meaning how may round balls of the diameter of the bore (or gage) it takes to make a pound. A 4 bore would be 4 balls to a pound, a 12 bore (12 gage) takes 12 round balls to make a pound.
I've held a 4 bore cartridge in my hand. Holding a metallic cartridge considerably bigger than a 12 ga shotshell made my shoulder ache.
I've read about Frederick Courtney Selous' exploits with the 4 bores many times. Never fails to make me grin, especially the one about his gun-bearer fully re-loading it instead of just replacing the misfired cap. Those old white hunters had some guts.
Now, 2 bores on the other hand.......