State Route 4 is a 168.85-mile (271.74 km) long state highway located in southern and western Maine. It is a major interregional route and the first such route to be designated in the state. The southern terminus is at the New Hampshire border in South Berwick, where it connects to New Hampshire Route 4, and the northern terminus is at Haines Landing on Mooselookmeguntic Lake in Rangeley. Major cities and towns along the length of SR 4 include Sanford, Gorham, Windham, Auburn and Farmington.

New Hampshire Route 4 is a 3.78-mile (6.08 km) state highway located in eastern Strafford County, New Hampshire. Its western terminus is at an intersection with New Hampshire Route 9 and New Hampshire Route 108 in Dover. Its northern terminus is at the Maine state border in Rollinsford, where it continues as Maine State Route 4.