The meridian 4° west of Greenwich is a line of longitude that extends from the North Pole across the Arctic Ocean, the Atlantic Ocean, Europe, Africa, the Southern Ocean, and Antarctica to the South Pole.
The 4th meridian west forms a great circle with the 176th meridian east.
It hits the north Wales coast just over 5 miles east of Bangor.
On its path south it passes a number of places including Snowdon (3 miles east) Barmouth (2 miles east) Aberdovey (2 miles east) Aberyswyth (3 ½ miles east) Ammanford (less than ½ mile west).
It leaves the south Wales coast 2 miles west of Swansea.
The cache is placed exactly on the 4th Meridian along the A487 between Tal-y-bont and Bow Street, there is a handy lay-by just north of the cache.