Route 4 is an east–west primary state highway connecting rural Litchfield County to the Greater Hartford area of the U.S. state of Connecticut. It runs 46.72 miles (75.19 km) from the town of Sharon to the town of West Hartford.

Route 4 begins at the junction of Route 41 and Route 343 in Sharon as a rural, minor arterial road, traveling through the towns of Cornwall, Goshen, and Torrington. From Torrington eastward, it becomes an urban principal arterial road for most of its remaining length. The road continues through Harwinton, Burlington, and Farmington. In Farmington, it runs along Farmington Avenue. Farmington Avenue (and Route 4) continues into West Hartford for another 0.9 miles (1.4 km) before becoming a local road. The rest of Farmington Avenue (no longer designated as Route 4) runs through West Hartford center up to downtown Hartford.

A 1.4-mile (2.3 km) section in Harwinton around the Cooks Dam area, and a 4.2-mile (6.8 km) section in Sharon, are designated state scenic roads.

Most of modern Route 4 was first improved as portions of various turnpikes in the 19th century. The section of modern Route 4 between Sharon and Cornwall Bridge was the Sharon and Cornwall Turnpike; from Cornwall Hollow to Torrington, it was part of the Goshen and Sharon Turnpike (which used West Cornwall Road and Route 128 between Sharon and Cornwall Hollow); the portion between Harwinton to Burlington was the eastern half of the Litchfield and Harwinton Turnpike (the western half is Route 118); and the portion from Farmington to West Hartford was part of the Farmington and Bristol Turnpike (which used George Washington Turnpike and Red Oak Hill Road between Burlington center and Route 10).

The road between Cornwall Bridge through Torrington to Collinsville in Canton (part of the Goshen and Sharon Turnpike and the Torrington Turnpike) was designated as State Highway 123 in 1922. The road from Collinsville to Farmington center was designated as State Highway 138 at the same time. Route 4 was created in 1932 from these two state highways, running as a continuation of old New York State Route 361 from Amenia, New York to Farmington at Route 10. In the Torrington area, Route 4 originally ran further north than it does today, along the current alignment of U.S. Route 202 from Torrington to Collinsville, then southward along modern Route 179 to Unionville, where it continued east on its current alignment. Route 4 was relocated in 1963 to its present day route through the area, taking over part of Route 72 and part of Route 116 (now Route 118.

The west end was truncated in 1966 to end at Route 41 and the section connecting to the state line was renumbered to Route 361 to match the New York route. New York has since turned its Route 361 over to the county. In the Hartford area, Farmington Avenue east of Route 10, from Farmington to downtown Hartford, used to be part of the original U.S. Route 6 alignment. When US 6 was rerouted in 1956, Farmington Avenue was assigned to an extended Route 4. The route designation was later truncated to end in West Hartford where state maintenance of the road ended.