State Route 4 is part of Maine's system of numbered state highways, running from the New Hampshire border in South Berwick to Haines Landing on Mooselookmeguntic Lake in the town of Rangeley. Route 4 is 168.7 miles (271.5 km) in length.

SR 4 was conceived in 1931 as a continuation of U.S. Route 4 in New Hampshire, hence its number. Although US 4 does come close to the Maine border, this continuation never came to fruition, as US 4 was routed south of Dover, New Hampshire to terminate in Portsmouth, New Hampshire. The intended connection is now indirect, facilitated by NH 4, NH 9, and NH 16 (the Spaulding Turnpike). Because the state of New Hampshire also numbered SR 4's 3-mile extension into New Hampshire as NH 4, there exists two routes in Dover numbered 4.

As it was first designated in 1931, SR 4 extended from its current southern terminus in South Berwick all the way to the Canadian border. By 1937, most of the routes that are cosigned with it had been designated, most notably U.S. Route 202. In 1946, SR 4 was truncated to its southern intersection with SR 16 in Rangeley, but was later re-extended along SR 16 to terminate at its current end at Haine's Landing in 1951 (this road between SR 16 and Haine's Landing had previously been designated SR 144). Most of former SR 4 between SR 16 and the Canadian border is now part of SR 27.

From South Berwick, Route 4 runs in a northeasterly direction through the town of North Berwick, bypassing the center of the city of Sanford to the east, and coming to an intersection with U.S. Route 202 in Alfred. Route 4 runs concurrently with U.S. 202 for 56 miles (90 km) to the city of Auburn, through the towns of Gorham, Windham and Gray.

North of Auburn, Route 4 runs north, staying near the west bank of the Androscoggin River, through the town of Turner, to Livermore, where it crosses the river to Livermore Falls. Route 4 continues north to an intersection with U.S. 2 in Wilton, and runs concurrently with U.S. 2 for seven miles (11 km) to Farmington.

Route 4 follows the Sandy River north of Farmington, through the towns of Strong and Phillips, and west to the river's source near Saddleback Mountain. The road turns north to Rangeley, where it turns west and runs concurrently with Route 16 at Oquossoc Route 4 meets Route 17 where Route 4 continues west to its end at Haines Landing on the shores of Mooselookmeguntic Lake.

State Route 4A is the designation for two separate, but closely located, state highways in southeastern Maine, both located in York County. Although they are not directly connected to one another, they are linked by their parent highway, Maine State Route 4, as well as U.S. Route 202, which runs concurrently with Route 4 during this stretch.

The southern segment of State Route 4A runs between the towns of Sanford and Alfred, and is multiplexed with another route over its entire 6.5-mile (10.5 km) length. It begins at an intersection with State Route 4 and State Route 109 south of town. Routes 4A and 109 run concurrently to the northwest, into the center of town, where they intersect with U.S. Route 202 and State Route 11. Route 11 splits off US-202 to join Route 109 to the northwest, while Route 4A splits off Route 109 to join US-202 to the northeast. Route 4A and US-202 run concurrently to the northeast, crossing into Alfred, before intersecting with Route 4 and State Route 111. Route 4A's southern segment ends at this intersection, US-202 turns north to join Route 4, and Route 111 begins to the east.

Route 4A's northern segment is the shorter of the two, but has not been overlaid with new routes like the southern segment has. Route 4A picks up at an intersection between US-202, Route 4, and State Route 35 in Hollis. Routes 4A and 35 share a brief concurrency before Route 4A splits off to the east, crossing State Route 117 in the process. Route 4A continues to the east, paralleling US-202, Route 4, and Route 117, which share a concurrency, and entering the town of Buxton. Route 4A then crosses over State Route 112 before coming to an end at the US-202/Route 4 concurrency.