A friend of mine called me out to his truck this afternoon and handed me this.  He thought it was a cannon ball but it had no hole for a fuse.

The ball is iron and appears to be cast.  It has the number four “4” imprinted on it and a small indentation beside the imprint.  Its about 4 inches in diameter and about 4 pounds.   The soda can is provided for reference on size only.

Does anyone know what this is?