I don't know about 'affordable,' but you could try Searcy Rifles, apparently 4-bore is one of the options they have on their lists for calibres, among other things like .577 Nitro Express and other cannons like that. Best browse their website while sitting down, otherwise you could lose your balance when you look at the pricelist.
'Affordable' and '4-bore' just don't sound like two words you could use in the same sentence. 4-Bore exists as an option on the Searcy Double rifles, but it adds a value of $85,000 to the overall price of the rifle.
- Dave.