The road begins to the north of Swansea at the junction with the A483 and heads westwards. It continues over the Loughor bridge into Bynea, Carmarthenshire and through the centre of Llanelli and traverses near the south coast of Carmarthen Bay until it reaches Pembrey. After Pembrey, the road bears northwards and continues in a northerly direction towards Carmarthen. At the A40/A48 junction at Pensarn, the A484 follows the A40 to the junction with the Capel Dewi Road and crosses the River Towy on Carmarthen Bridge and continues through eastern Carmarthen to Bronwydd Arms. Here it takes a northwesterly direction to Cynwyl Elfed, then northwards again. After the village of Llangeler, it turns west and continues westwards until the junction with the A487 in Cardigan.
Prior to the Construction of the "Llanelli Link Road", the road diverged from the A483 started in Cadle at a fork, where Carmarthen Road and Pontarddulais Road diverged. The A483 continued, along Pontaraddulais Road, whereas the remainder of Carmarthen Road carried the start of the A484. The section of road was declassified and is in effect a residential cul-de-sac, since the original start of the road is now only open to buses. From the junction with Ystrad Road, the A484 continued along what is now the B4620 to Loughor.
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