Concurrency with Route 225 Route 4 begins at Route 2's Exit 131 interchange in Lexington, just west of the border with
Arlington. This interchange also serves as
Route 225's eastern terminus, and the two routes begin overlapped with one another. Routes 4 and 225 proceed northward on Watertown Street, turning onto
Massachusetts Avenue and meeting
Route 2A in East Lexington, briefly sharing pavement with it. The two routes continue through the center of Lexington, where Mass Ave splits to the west, and Routes 4 and 225 continue northwest on Bedford Street. The concurrency passes near North Lexington and interchanges with
Interstate 95 (
Route 128) at Exit 49, before continuing northwest into the town of
Bedford. Routes 4 and 225 pass into the center of town, meeting
Route 62 and running concurrent through the town center. After passing through town, Route 62 splits to the west, while Routes 4 and 225 turn northward, splitting themselves soon thereafter; Route 225 to the northwest and Route 4 to the north.
Northern segment Route 4 proceeds northward out of Bedford by itself, where it begins to parallel US 3 as it passes into
Billerica. Route 4 does not intersect any other numbered highways in the town, but does intersect two local roads - Concord Road and Treble Cove Road - which provide nearby access to US 3, which lies to the east. Route 4 continues north into
Chelmsford, where it meanders towards the center of town. Near the center, Route 4 intersects the northern end of
Route 27 at a rotary-type intersection, and then crosses
Route 110, very briefly sharing pavement. Continuing to the north, Route 4 has a half-interchange with
Interstate 495 at Exit 87. 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. Route 4 continues to the northwest, where it comes to a rotary interchange with US 3 at Exit 84, crossing over the freeway, and continuing another half-mile to the northwest where it ends at an intersection with Route 3A in
North Chelmsford. ==Major intersections==