|Length:||6.2 miles (10 km)|
|Meets:||A4054, B4255, A469, B4252, A4049, B4251, A4048, B4251|
|Old route now:||B4255|
|Route outline (key)|
The B4254 connects the Taff and Sirhowy Valleys in South Wales. It has a complicated route and history.
The road's current western end is on the A4054 in Edwardsville. It heads east to Trelewis where it meets the B4255 and multiplexes with it south through the village before regaining its number and heading east once more.
Originally the B4254 started here in Trelewis and headed east, with the road to Edwardsville being the B4255. By the 1930s the B4254 had been extended north along a dead end to Bedlinog and in 1935 the roads north of Trelewis swapped numbers, with the B4255 going to Bedlinog and the B4254 to Edwardsville, as they still do.
Heading east on its 1922 route, the B4254 goes through Gelligaer and under the Rhymney Line at a narrow bridge controlled by traffic lights. It then reaches its original eastern terminus at a traffic-light-controlled crossroads, where the other three directions are numbered A469.
The next section is complicated and requires four different road numbers in under a mile. The B4254 carries on ahead multiplexing with the A469. It crosses the Rhymney River and turns right, now on a useless multiplex along the B4252 (that road was here first and it was not deemed necessary to change its number). The B4254 does reappear by turning left onto St David's Road before there is another multiplex south along the A4049 between two sets of traffic lights.
The B4254 now continues east bypassing Blackwood to the south. When it was originally extended east it took over a slightly rerouted B4253 along Fair View and Bloomfield Road but it now follows a new road further south. It turns right at a roundabout before crossing the B4251 at traffic lights. It then crosses the Sirhowy River before ending at a roundabout where it meets the A4048 and a spur of the B4251.