Construction of the last stretch of four-lane Highway 36 in northern Missouri begins today with a groundbreaking ceremony near Hannibal. The event marks the beginning of work on 52 miles of road from Macon to the Illinois line. It will cost about $100 million to expand the present two lanes to four. Work is to be finished in four years. Four of the six counties through which the improved road will pass have approved a special sales tax to raise $43 million for the project.