U.S. Route 4 (US 4) is a part of the United States Numbered Highway System that runs from East Greenbush, New York, to Portsmouth, New Hampshire.

The portion of the route between Waterford and Whitehall is part of the Lakes to Locks Passage, an All-American Road.

US 4 was extended southward to its present terminus in East Greenbush as part of the 1930 renumbering of state highways in New York.

The concurrency was split into two shorter overlaps when US 4 was realigned in the 1950s to follow its current alignment between Bemis Heights and Schuylerville.

At Congress Street, in the middle of the Central Troy Historic District with St. Paul's Episcopal Church on the corner, it intersects NY 2.

They run along the riverbank for seven miles (11 km) to the city of Mechanicville, where the routes cross over the Anthony Kill and intersect NY 67 in the central business district.

US 4 and NY 32 continue on, passing through the nearby village of Stillwater before splitting at Bemis Heights, a small community five miles (8.0 km) northeast of Mechanicville.

Past the park, the route passes by Gerald B. H. Solomon Saratoga National Cemetery before rejoining NY 32 at the southern edge of the village of Schuylerville.

[9] US 4 begins to run along the Champlain Canal after Schuylerville, passing through the hamlets of Northumberland and Starks Knob.

At the same time, Route 44 was extended southwest from Stillwater to Clifton Park via Mechanicville and west from Schuylerville to Saratoga Springs.

Routes were first posted in New York in 1927, US 9 was restored to its originally planned alignment between Albany and Glens Falls (via Waterford and Mechanicville) while the US 109 designation went unassigned.

[16] In the 1930 renumbering of state highways in New York, US 9 was realigned to follow its modern routing between Albany and Round Lake.

The portion of US 9's former routing between Waterford and Mechanicville as well as all of US 9E north of US 20 became a southward extension of US 4, which left its original alignment in Hudson Falls and followed a previously unnumbered riverside highway south to Northumberland.

[17][18] Virtually all of US 4 south of Hudson Falls initially overlapped other routes, all of which were assigned as part of the renumbering.

The realignment moved the northern end of the overlap southward to the junction of US 4 and Winter Street in North Greenbush.

[21][22] In the mid-1950s, work began on a project to upgrade a preexisting riverside highway between Bemis Heights and Schuylerville.