Junction 2 (restricted access): A205 South Circular, A406 North Circular, A315 Chiswick, A4.
Junction 1: No exit
The M4 ends six miles (10 km) west of central London where it becomes the A4.
Notes
In the construction of the section around Cardiff, when a section between Bridgend and Port Talbot already existed, numbers were set aside for intermediate interchanges. 33 had been set for the link road to the west (A4232), 32 for the A470, with 31 intended for the A469 to Caerphilly, and 30 for the great "Hook Road" - a plan which was later shelved. Junction 30 was later added to serve the link road in the east, which will eventually form a ring road from junctions 33 to 30, via Cardiff Bay. Number 31 remains unused as of 2005, and there are no immediate plans to build a junction with the A469.
Contrary to popular belief, junction 39 does exist, however, it consists of a single slip road completing the missing movement onto the westbound carriage way from the A48 at junction 38, thus there is no exit from the motorway at this junction.
Junction 41 refers to two different junctions in the two different directions, their movements not complementary. In the westbound direction, 41 is indicated as a spur leading to the roundabout in Briton Ferry formerly known as 41A, and the original bridge over the River Neath, which would allow access onto the stretch of the M4 from junction 43 westward. In the eastbound direction, 41 is indicated as an exit-only route to the A48 towards Port Talbot. As a result, it is possible in both directions to travel almost 2 miles (3 km) in the same direction having both joined and subsequently left the motorway at "junction 41".
