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Easily one of the most varied motorways in Britain, from urban chaos in west London and Heathrow Airport, out through the rolling hills of Gloucestershire, across the Severn Estuary and through industrial South Wales.
Before the M4 Severn Bridge was built, there was no road bridge over the Severn south of Gloucester, leaving traffic from South Wales to South England to drive the A40 or take the A4 and then a ferry or a motorail crossing at Bristol.
Recently, the Second Severn Crossing was built and the M4 moved there, creating a new motorway, the M48, across the old bridge. The M49 was also added to cut congestion at the M4/M5 junction. Because of these multiple motorway connections, along with the M32 Bristol Spur, the M4 holds the UK record with five consecutive motorway interchanges.
Until recently the M4 was in two sections, joined by six miles of A48 at Briton Ferry due to the marshy land. This was eventually connected up with a large and horrendously expensive viaduct over the top of the whole thing.
Factfile
| Start | London (A4) |
| Finish | Pont Abraham (A48) |
| Passes | Hounslow, Heathrow  , Slough, Maidenhead, Reading, Newbury, Swindon, Chippenham, Bristol, Newport, Cardiff, Bridgend, Port Talbot, Swansea |
| Length | 193 miles |
| Terminates | M48, M49, A417/A419 |
| Spurs | M32, A48(M), A308(M), A404(M) |
| Meets | M5, M25, A329(M) |
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With thanks to Michael Young, Dave Ross, Chris Jackson, Steve, Chris McKenna, Chris Boots, Tim Lidbetter, Neil Padgen, Jonathan, Chris Clifford, Chris (a different one) and Martin Wareham for information in this section.
