The B4640 in northern Hampshire follows a part of the pre-bypass route of the A34 south of Newbury. It is not known why this number was chosen as it is out-of-zone and bears no relationship to the former number of the road.
The road starts at a dumbbell GSJ on the A34 which also provides access to Tot Hill Services and an unclassified road to Burghclere. We follow a dead-straight route northnortheastwards before bearing to the right on Newtown Common. After skirting the edge of Newtown itself we cross the River Enbourne to enter Berkshire and end immediately afterwards at a roundabout on the A339. That road now continues ahead along the ex-A34 into Newbury.