The highway began as a trail parallel to a historic branch of the Canadian Pacific Railway that was built in the late 1800s connecting Lethbridge to Great Falls. It had been named the Sunshine Trail by 1912, and developed into an all-weather gravelled road by the 1930s. Paving and a realignment to eliminate curves was completed by 1947, and in the late 1980s Alberta Transportation announced plans to twin the entire length as part of upgrades to the CANAMEX Corridor south of Calgary that included Highways 2 and 3. An expressway bypass of Milk River completed all work in 2009. A bypass of Lethbridge at the highway's northern terminus is also proposed; it would link Highway 4 to a newly aligned Highway 3 north of the city, making Alberta's portion of the CANAMEX Corridor free-flowing from Coutts to Fort Macleod.