King's Highway 4, also known as Highway 4, is a provincial highway in Ontario, Canada. It starts at Highway 3 in Southwold. It then continues northward until ending at Highway 8 in Clinton. It is 100.8 kilometers (62.6 miles) long. The highway passes through Elgin County, Middlesex County, and Huron County. It also passes through the major cities of St. Thomas and London.