The B4581 was a short road to the east of Aberystwyth, acting as a makeshift bypass for the town. The road was originally unclassified but gained its number in 1935. Owing to its importance it became an extension of the A4120 in the late 1960s and has since been improved.
The road started in Penparcau, more-or-less at the point where the A487, A4120 and B4340 all met. It headed north (the first section has now been bypassed) and soon crossed the Afon Rheidol. Continuing on it went over two level crossings in quick succession (the first carrying the narrow-guage Vale of Rheidol line, the second the main line to Machnylleth), then ended in the square at Llanbadarn Fawr on the A44/