|From:||New Mills (SJ099011)|
|Length:||7.2 miles (11.6 km)|
|Meets:||B4389, B4385, A483|
|Route outline (key)|
The B4390 is a rural B-road in mid-Wales along the valley of the River Rhiw in a generally easterly direction. It was originally unclassified but gained its number in 1935.
The road starts in New Mills as a straight-ahead continuation of the road from Bettws Cedewain and Tregynon, where that road (the B4389) turns sharp left to continue northwards towards Llanfair Caereinion. From here the B4390 winds its way along the northern bank of the Rhiw for some 10.5 km through Manafon and Pont-y-fridd – with mostly wooded hills close by on either side – to Berriew. This village, as its Welsh name of Aberriw suggests, lies close to the mouth of the River Rhiw, which flows into the River Severn a short distance away.
From here, after passing through the village in the brief company of the B4385, a final near-dead-straight kilometre across the flat valley floor of the Severn and over the Montgomery Canal brings the route to its conclusion on the A483. Welshpool is not far to the left.