The B4523 was a short link road in the Rhondda Fawr valley. Owing to its short length the history of the road is not fully known. However, it had definitely gained its number by the mid-1980s. In more recent years the road's bridge over the Rhondda has been removed with the construction of the present route of the A4058 and so the former B4523 has been destroyed as a through route. Only its eastern end is still classified, as part of the B4278.
The road started on the A4119 Dunraven Street in Tonypandy and headed northeast along Bridge Street to cross the Rhondda and the railway line on bridges that no longer exist. It then met Trealaw Road, the original route of the A4058, and turned right along that road, ending where it met the later A4058 on Miskin Road.