could be melted silverware. bunch of silverware that was melted in a fire or something. a jewler would be able to tell you if its silver or not. Check if its solder or lead first...you could cut a piece off pretty easy if it was.
Do you know if the ship you are diving on burned? Are you in fresh or salt water. Silver comes out of the ocean pretty black. I'd sure test it for silver. 4 pounds of silver will fill your tank a couple of times, that would be a great find.
Is it magnetic? Have you tried clipping off the corner with snips to see if its possibly lead or solder? We can make a thousand guesses as to what it is,but without some info beyond the weight of the object,its anyones guess what that is/was.