It is indeed a 4 bore, the rifle belongs to the collection in the Royal armouries Museum in Leeds, West Yorks, England. The bearded Gent holding it is one of the curators of the museum.

Anyone ever on a visit to the UK would be well advised to pay a visit to the museum, the collection is vast and amounts to many thousands of weapons both edged and firearms from the earliest wheel locks through to the most upto date military weapons from all over the world.

Also housed within the bowels of the museum is the former "Pattern Room collection" formerly the property of the Ministry of Defence, now called the National firearms collection. This collection contains samples of virtually all military weapons ever made both handgun and long arms. Entry to the museum is free, definitely worth a look.