The B4577 is a cross-country B-road in central Cardiganshire. The road was originally unclassified but gained its number in 1935.
The road starts on the B4578 at Tyncelyn, just a house's distance from that road's junction with the A485. We head west along the usual sort of road for this part of Wales: too narrow to be S2 but too wide to be S1. After crossing the wooded valley of the Afon Aeron we climb steeply then continue across undulating fields, with a view of the valley opening out to the left.
Presently we run through the scattered settlement of Penwch, at the end of which the B4576 comes in from the left. The B4577 is the dominant number in a multiplex ahead into Bethania; that road turns off to the right on the near side of the village. After reentering open country there's a short wooded section and we reach Cross Inn, apparently the name of the settlement surrounding the pub of the same name, where we give way to the B4337.
We continue west. A few more bends take us into Pennant, where we give way to an unclassified north-south route; indeed the village centre lies to the left. We go over a ridge and the sea becomes visible ahead. A sharp left-hand bend takes us onto a descent down to sea level and the road ends at a T-junction on the A487 in the coastal village of Aberarth.