Hallo skull,
with a 90% assurance your ball is a 6 pounder. I do not have any european caliber lists before 1835. According to the Prussian Manual of 1835 the dia of an english 6pounder ball is 3,6? prussian inches. 1 prussian inch is 2,61545 cm. So, an english 6pounder ball would have a dia of 9,4156 cm = 3,707 inch. Looking at your ball one can see that it has been cleaned and that only the most resistant oxide layers are left. This could account for the small discrepancy in measurement. In the list of 1835 there is no mention of a 4pounder cannonball. I expect that there will not have been any great changes in cannon calibers from the 18th to the early 19th century. So, I assume that my 1835 list holds good also for the 18th century. English calibers for cannon of that time were 3,6,9,12,18,24,32 and 42 pounders.
Regards,
Bellifortis.
