|Length:||1.3 miles (2.1 km)|
|Met:||B4320, B4322, A477|
|Now part of:||A4139|
|Route outline (key)|
The original B4321 was a short link road in Pembroke.
It started on the B4320 at the west end of Main Street in the centre of Pembroke, within sight of the castle. It headed north along Northgate Street to cross a bridge over the Mill Pond before continuing on through the suburbs parallel to the railway line. It ended on the A477 in the Waterloo part of town.
The road had been upgraded to Class I status by 1932. It became the A4139, which it remains.