IL 4 coincided with most of the Pontiac Trail and closely paralleled the Chicago and Alton Railroad tracks running from Chicago to East St. Louis. The roadbed for IL 4 was prepared in 1922 by teams of horses dragging equipment behind them. Laborers received 40 cents per hour for performing backbreaking labor on the roadbed. In 1923 in Bloomington-Normal, concrete was poured along the road's path along much the same route US 66 would take on its original route through the area. By 1924, IL 4 was almost entirely paved between Chicago and St. Louis. Construction on the few remaining parts of US 66 in Illinois began in 1926.