Much of it, north of Lexington, runs parallel to U.S. Route 3 (the Northwest Expressway).

The concurrency passes near North Lexington and interchanges with Interstate 95 (Route 128) at Exit 49, before continuing northwest into the town of Bedford.

Route 4 proceeds northward out of Bedford by itself, where it begins to parallel US 3 as it passes into Billerica.

This interchange provides southbound-only access to I-495 and is a northbound-only exit from the freeway.

Access to I-495 northbound is available via Route 110 East, at the nearby Exit 88 interchange.