The M4 motorway links London and south Wales and is part of European route E30. Entering Wales via the Severn Bridge (nprn 43161) and the Wye Bridge (nprn 91661), both opened in 1966, it was originally planned to terminate at Tredegar Park, west of Newport. By 1977, the western end of the M4 was at St Mellons, on the eastern outskirts of Cardiff, but following extension of the motorway westwards from Castleton round the north of Cardiff in 1980, a short spur section of the original M4 between junctions 29 and 29A was re-designated A48(M).

Now following the route of the A48 trunk road (nprn 412594) across south Wales, the M4 currently ends at Pont Abraham, near Ammanford, where it joins the A48. The M4 incorporates the Port Talbot by-pass, an elevated four-lane stretch of motorway opened in 1966 and the 4 mile-long Morriston by-pass built in 1972. Following the opening of the Second Severn Crossing (nprn 405) in 1996, the section between Olveston (junction 21) and Undy (junction 23) was renamed M48 and the SSC now carries the M4 between England and Wales.

The National Grid Reference given, ST 43600 87920, is the point where the M48 forms a junction (no.23) with the M4, north of Undy in Monmouthshire.

B.A.Malaws, RCAHMW, 04 September 2012.