U.S. Route 4 crosses the Connecticut River into New Hampshire in the community of West Lebanon, where it immediately intersects Route 10 which runs parallel to the river. US-4 turns south onto Route 10, and the two routes turn south, meeting Route 12A before turning towards and interchanging with Interstate 89. At this point, Route 10 joins the I-89 freeway southbound, while US-4 continues east into downtown Lebanon. The road crosses Route 120, continues east and interchanges with I-89/Route 10 again. US-4 continues east away from the freeway near Mascoma Lake, where Route 4A splits off to the southeast. US-4 continues east through Enfield and into Canaan, where it meets the southern end of Route 118. The road turns to the south at this point, passing through Grafton and Danbury, where US-4 meets the west end of Route 104 and continues south into Andover. In Andover, US-4 turns back to the east and meets Route 11 near Route 4A's eastern terminus. US-4 and Route 11 run concurrently through Andover for about before splitting, Route 11 to the northeast and US-4 to the southeast. US-4 enters the town of Salisbury and crosses Route 127, before continuing into Boscawen and intersecting with U.S. Route 3. US-3 and US-4 share a short concurrency, before US-4 turns east to interchange with Interstate 93 at Exit 17. US-4 joins I-93 southbound, and runs along the freeway until Exit 15E in Concord. At this interchange, US-4 leaves I-93 and joins Interstate 393 and U.S. Route 202 which run eastbound out of the city. The two U.S. routes overlap I-393 to its terminus in the northern corner of Pembroke. I-393 then ends, and US-4/US-202 merge onto Route 9 eastbound through Chichester and into Epsom. The road crosses Route 28 at the Epsom Traffic Circle, then continues east and intersects Route 107, forming a long four-route concurrency into Northwood, where Route 107 splits off to the northwest. US-4, US-202, and Route 9 continue through Northwood, and US-202 and Route 9 split from US-4 at an intersection with Route 43. US-4 continues east, meeting the west end of New Hampshire Route 152 and proceeding into Nottingham and then into Lee, where US-4 meets Route 125 at a two-lane roundabout. After leaving this interchange, US-4 crosses into Durham and becomes a semi-limited-access highway. US-4 has a partial eastbound interchange with Route 155 and a diamond interchange with Route 155Adown the road, providing access to the University of New Hampshire campus in Durham. US-4 has one more interchange, with Route 108, before becoming a full-access highway again. US-4 continues east towards the coast and crosses the tidal Bellamy River to enter Dover, then interchanges with the Spaulding Turnpike. US-4 joins the Turnpike southbound, closely paralleling the Maine state border and crossing the Little Bay Bridge into the town of Newington before continuing into the city of Portsmouth. US-4 terminates just south of the Maine state line at the final southbound interchange with Interstate 95, where the Turnpike splits to merge with I-95 South, and Route 16 continues south to end at the Portsmouth Traffic Circle, providing access to I-95 North and U.S. Route 1 Bypass.
New Hampshire Route 4A is a long route between Lebanon and Andover, New Hampshire, serving as a shortcut around several villages on U.S. Route 4. Until Interstate 89 was built in the early 1970s, this was part of the main route between the Lebanon-Hanover area and the southeastern portion of New Hampshire. Today, traffic is very light on this road. NH 4A is signed as a north-south highway, although its orientation is more southeast-northwest. The northern terminus is in Lebanon at US 4, near the western tip of Lake Mascoma. The southern terminus is in the town of Andover at New Hampshire Route 11, about 3/4 mile southwest of its intersection with US 4. This highway is locally named the 4th New Hampshire Turnpike. Route 4A is an alternate route of U.S. Route 4, and not of New Hampshire Route 4, a completely different route located in Dover.
