The B4519 is a fairly short B-road on Mynydd Epynt in mid-Wales. The road was originally unclassified but gained its number in the mid-1930s.
Starting on the A483 on the eastern edge of Garth, the road heads south, quickly going under the Heart of Wales Line then over the River Irfon before starting to climb out of the valley. The gradient becomes steeper as the road continues. The centre line disappears and the road climbs along the edge of a hill before continuing along a ridge and crossing a cattle grid to enter the MoD Sennybridge Training Area. The road remains open even when the red flag is flying, but in such cases people should not leave the road. There are a number of side roads leading further into the training area.
After passing a lay-by the road levels out and continues across the moor along a winding, unfenced route. We pass the Drover's Arms – once a pub but now used for military exercises – and the visitor centre in the former hamlet of Pentre Dolau Honddu. After a lay-by we cross another cattle grid to leave the training area and then descend straight ahead. The road ends at a T-junction on the B4520 just north of Upper Chapel.