Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Route 4 is an 18.3-mile (29.5 km) long state highway in northeastern Massachusetts. It runs south to north, serving many of Boston's western and northwestern suburbs, from an interchange with Route 2 in Lexington northwest to an intersection with Route 3A in North Chelmsford. Much of it, north of Lexington, runs parallel to U.S. Route 3 (the Northwest Expressway). The entire route is contained within Middlesex County.