Checked the documentation, and the only reference I can find anywhere to 4-bore penetration of only 3 inches is in mbogo's post!

I did find a paragraph on the 8-bore which might interest you though, penned by R.C.F. Maugham, inveterate big game hunter, and His Majesty's Consul-General at Beira, Portugese East Africa; (writing just after the turn of the century):

"....no hunter in his right senses should place his blind trust in such a weapon as even the .450 cordite rifle to extricate him from the thousand and one dangers by which he is liable from day to day to be confronted in Africa. I therefore unhesitatingly advocate, in addition to the most perfected type of modern high-velocity weapon which we are ever likely to see, the inclusion of a sound, heavy rifle intended for use at close range, the shock of whose bullet, even if it should not instantly kill, will assuredly be sufficiently great to disable or turn a charging beast."
His recommendation?
"..a heavy eight-bore, which I look upon as the ideal weapon par excellance for this purpose".