I can take a semi-educated guess for you in building a 4bore rifle.
I think most (maybe all?) of the 4 bore singles used in Africa, aka Francis Selous, were smoothbore fowlers shortened in the barrel length for handiness in use. Selous' 4bore single was a Dutch 4 bore fowler, I'm quite sure - read that somewhere. I do not know the measurements of the breeches.