The M4 motorway is a motorway in Great Britain linking London with Wales. It is part of the unsigned European route E30. Other major places directly accessible from M4 junctions are Reading, Swindon, Bristol, Newport, Cardiff and Swansea.

The M4 runs close to the A4 from London to Bristol. After crossing the River Severn it follows the A48 through Wales, until its termination just north of Pontarddulais.