|Location Map ( geo)|
|Distance:||3.1 miles (5 km)|
|Meets:||A469, B4256, A469, A465|
|Former Number(s):||A469, A465|
|Old route now:||A469|
|Route outline (key)|
The B4257 is a short B-road in the Rhymney Valley.
In 1922 it started on the A469 in the centre of Rhymney and headed northwest along Coronation Terrace out of town. It crossed the Rhymney River and continued north via Bute Town to end on the A465 to the west of Llechryd.
The building of the A469 Rhymney bypass took over the northern section of the B4257 but in return it gained the whole of the other road's pre-bypass route. Thus the B4257 now starts on the A469 in Pontlottyn. It heads east and immediately goes under the Rhymney Line and over the Rhymney River before turning northwards.
After meeting the B4256 at two separate junctions a few hundred yards apart (to allow access from both directions of the B4257) the road goes through the centre of Rhymney. At the northern end of the High Street it reaches its original southern terminus; the old road still exists as a spur as far as the bypass. However, the mainline now continues ahead out of town. It TOTSOs left in Llechryd where it meets the original line of the A465 before ending shortly afterwards at the southern roundabout of a Dumbbell interchange on the current route of the A465 at the northern end of the A469 (and original northern end of the B4257).