|To:||North Cwmbrân (ST299984)|
|Length:||6.6 miles (10.6 km)|
|Former Number(s):||B4244, A4042|
|Route outline (key)|
The route starts on the A4042 at the Old Green Interchange in Newport. From here it heads north along Malpas Road to cross the M4 at J26, after which it becomes D2 (albeit with numerous sets of traffic lights) as it runs through Malpas itself. A roundabout is met where the D2 road turns right to rejoin the A4042; this is a spur of that road. The A4051 continues northwards along the S2 Cwmbrân Drive. This is an apparent New Town distributor, following a relatively straight course through Cwmbrân, and as such is of higher quality than the D2 section through Malpas.
There is a short D2 section past Cwmbrân town centre, after which the road narrows once more and continues north, crossing the railway line and ending at a further roundabout back on the A4042 at Panteg, on the northern edge of Cwmbrân and southern edge of Pontypool.
Originally the route consisted of a shorter parallel loop to the A4042 as an upgrade and realignment of the B4244. It branched off the original A4042 Newport Road at Llantarnam and comprised Llantarnam Road, St Davids Road, and Avondale Road in Cwmbrân, then Station Road, Coedygric Road, and Stafford Road in Pontypool before terminating on the A472. The Cwmbrân Drive section was opened in the 1980s along the route of the former Monmouthshire Eastern Valleys railway line and the original A4042 through Newport was reclassified A4051 on the opening of the Brynglas Tunnels and Malpas Road Relief Road in the 1990s (the southern portion of which also used the Eastern Valleys railway alignment).