Route 4 is a 28.3-kilometre (17.6 mi) long provincial highway located entirely in York County, New Brunswick, Canada.

The highway begins on the Saint Croix – Vanceboro Bridge between the cities of Vanceboro, Maine and Saint Croix, and travels east to an interchange with Route 3 in Thomaston Corner.

Route 4 begins on the Saint Croix – Vanceboro Bridge over the Saint Croix River, which serves as both the boundary between Maine and New Brunswick and the border between the Eastern Time Zone and Atlantic Time Zone,[2] traveling east through the customs station.

Exiting Saint Croix, the highway crosses Route 630 as it begins to parallel a rail line belonging to the New Brunswick Southern Railway through rural York County.

The line was deemed surplus with the construction of the parallel European and North American Railway in the late 1860s, immediately to the south, and it was later converted to a road.