|To:||Three Cocks (SO166370)|
|Length:||2.1 miles (3.4 km)|
|Route outline (key)|
Starting on the A479 in the Brecknockshire town of Talgarth, the road heads north along New Street, a new alignment avoiding the narrow roads leading out of the Market Place which is the road's original western end (the A479 is now also on a new route but that came later).
The A4078 soon reaches Hay Road, its old route, and remains a high-quality road as it continues along the edge of the Wye Valley, with hills to the right and excellent views to the left. The road leaves the Brecon Beacons National Park at the next junction and continues along a largely straight road. It bends sharply left at almost the last minute, ending a couple of hundred yards further on at a T-junction on the A438.