He's of 'the firebrand's' line. Meet William Lyon Mackenzie King IV, who came onto the scene last Friday, but as yet has worked up no interest in the election. The 7-pound, 9-ounce descendant of 'firebrand' William Lyon Mackenzie is the son of Dr. and Mrs. W. L. M. King, a great nephew of Canada's long-time prime minister, and grandson of Surgeon-Lt. King, who went down with the destroyer HMCS St. Croix during World War II.