After crossing the River Severn, toll-free since 17 December 2018, the motorway follows the A48, to terminate at the Pont Abraham services in Carmarthenshire.

The major towns and cities along the route—a distance of approximately 189 miles (304 km)—include Slough, Reading, Swindon, Bristol, Newport, Cardiff, Bridgend, Port Talbot and Swansea.

[1] The Chiswick flyover, a short section of elevated dual-carriageway, not originally classed as a motorway, opened in 1959 to reduce the impact of traffic travelling between central London and the west.

The Ministry of Transport originally intended that the M4 would terminate at Tredegar Park west of Newport, and following the creation of the Welsh Office that the Government became committed to a high-standard dual carriageway to Carmarthenshire.

[5] The English section of the motorway was completed on 22 December 1971 when the 50-mile (80 km) stretch between junctions 9 and 15 Maidenhead and Swindon) was opened to traffic.

At the same time, the original route over the Severn Bridge was redesignated the M48, and the M49 was opened to link the new crossing with the M5 at Avonmouth.

[9] The scheme was formally opened on 25 January 2010 by Ieuan Wyn Jones the Deputy First Minister for Wales.

[12] The £65 million scheme included work on the Mereoak roundabout and part of the A33 Swallowfield Bypass near Shinfield, and also the conversion of the two existing bridges, one of which is available only to pedestrians and cyclists and the other to buses.

[13] It also involved the movement of the local Highways Agency and Fire Service offices, and the construction of a long footbridge network, a new bus lane and a new gyratory.

[14] In April 2008, the decision to preserve a rare Vickers machine gun pillbox and turn it into a bat roost was announced by the developers.

After 1996, the tolls were equal westbound-only on both bridges, and rose steeply after 2000 to a peak of £6.70 for cars in 2017, leading to protests from Welsh businesses.

[17][18][19][20][21][22] Maintenance of the Second Severn Crossing and the 123 miles (198 km) of motorway in England is the responsibility of National Highways.

With a length of 32 miles (51 km), on completion it became the longest smart motorway scheme in the United Kingdom.

The lane which had no intermediate exits was for use by buses, coaches, motorcycles, emergency vehicles and licensed taxis but not mini-cabs.

When driving in heavy rain drivers notice a reduction in road spray from other vehicles and improved visibility.

[40] The elevated section of the M4 in West London, built in the 1960s, is mostly directly above the A4 and extends over parts of Brentford's Golden Mile.

It is a back entrance to RAF Welford, a Second World War airfield and now an RAF/USAF military installation mainly used for storing munitions.

[45] A freedom of information request in 2010 to Cardiff Council shows that whilst the land that would enable this junction should continue to be strategically protected, the decision to formally abandon the proposed Junction 31 Thornhill was made in October 2007 and there had been no subsequent mention of it in Cardiff Council Strategic or Planning meetings since.

Junction 27 (High Cross) is a normal grade-separated roundabout junction, but has severe space constraints: traffic joining the motorway must initially travel in the opposite direction to the intended direction of travel, before making a sharp left-hand turn from the slip road onto the motorway.

It lacks continuous hard shoulders, has closely spaced junctions with sub-standard slip road visibility and narrows to a restricted two lane section through the Brynglas Tunnels.