Gents:
My 4-bore is a .975" diameter for a bore size of 5-bore as are most 4s. I have only seen one double that is a true 4 bore and a few singles. too. In fact the 4-bore 4 may be up for sale next year (as well as a Rodda). Which ever comes up first I have first refusal and, if so, I will put my Hughes up for sale to upgrade a bit.
Cal