The A4153 was a short-lived A-road on the border between Wales and England.
The road started on the A438 on the opposite bank of the River Wye to Glasbury. It immediately crossed the river and then ran downstream along the valley. After leaving Glasbury it went through Llowes and Clyro, from where the B4351 crossed over to Hay on Wye. A couple of miles further on the road entered England at Rhydspence before returning to the A438 just to the north of Whitney Bridge, where the road ended.
The A4153 was originally the eastern end of the B4350 (and, before 1935, the whole of that road), whilst the main A438 went through the centre of Hay on Wye and crossed the toll bridge at Whitney. Around 1960 it was deemed appropriate to bypass Hay with a Class I road; this became the A4153. However, it was soon decided that the A4153 was the more appropriate route than the A438 through the town and so the latter was downgraded and the A4153 became the mainline of the A438, which it remains. The only question is why this was not done earlier.