However, by the mid-19th century, some trusts were badly managed or abused, exacerbating rural poverty and in part leading to the Rebecca riots in the 1840s, some of the earliest of which were on this road, particularly at Efailwen in the Cilymaenllwyd Community.

The river silted up and larger vessels could no longer reach the port, which had largely become inactive by the early part of the 20th century.

The road passes through Pen-y-bryn in Bridell parish, crossing the River Plysgog, and climbs Rhoshill, crosses the B4332 Eglwyswrw-Cenarth road, then climbs the northern slopes of the eastern end of the Preseli Hills through Blaenffos village, bridging the River Nevern (near its source) at Riverlea, to Crymych village.

After passing the Folly Farm Adventure Park and Zoo and continuing through Begelly (Welsh: Begeli) it crosses the A477 road on a roundabout.

Continuing on the A478 and passing through Pentlepoir, Wooden and Moreton at the roundabout at Twycross it reconnects with the other end of the B4316; the preferred route to Saundersfoot.

After its junction with the A4139 and Tenby High Street, the A478 runs to the side of North Beach and terminates at the harbour by the town wall.