"barrel seems to be around 23 mm diameter so inch pincher will deny its a 4 ga."

Having a 4 bore chamber makes cases easier. A 6 bore barrel, 23mm being .019" under 6 bore of .919".
Not knocking you saying it is a 4bore, lancaster, as that is the case it is chambered for.
If shooting balls (deer, bear, hogs, trucks), something around .905 would be about perfect and weigh around 1,000gr., maybe a bit more or less, depending on alloy.
What a nice gun to have and play with.