It is the cypher of Charles/Carlos IV of Spain who ruled from 1788 to 1808, Allen. This is a photo of the cypher as it appears on a bronze 12-pounder on display in Leutze Park, Washington Naval Yard.
U.S. Navy
This information on the cannon displayed at Seaside Park, Bridgeport, CT was obtained from "The Chief of Ordnance Report of 1901."
"Detailed description of the captured Spanish cannon distributed to the several States under orders of the Secretary of War dated July 1, 1899."
Bronze 4.8-inch Muzzle Loading Rifle
On base ring No. 3559 Sevilla, 13 de Deciembre de 1794
On surface of piece, Reinforce: "IV."
On surface of piece, Chase: "Labadaco."
On left trunnion: "Cobre de Mexico y Riotinto."
On right trunnion: "P 3600."
Labadaco is the name the piece was given, but I haven't had any luck trying to translate it to English.
flickr, JamesPolk
flickr, kgandarillas