Junction 8/9 near Maidenhead, Berkshire, and High Wycombe, Buckinghamshire is the only one in the UK with dual numbers. This arose at the time when the M4 turned north near junction 8, where it met the A308, and headed for the original junction 9, where the motorway ended at a roundabout interchange with the A4.
Four level stack interchangesThe M4 has two of the three four-level stack interchanges in the UK, including the first UK example at junction 20, the Almondsbury Interchange with the M5. The other is junction 4b with the M25.
Membury services is a motorway service station on the M4 motorway, located on the original site of RAF Membury in the civil parish of Lambourn in the English county of Berkshire, close to the border with Wiltshire, 4 miles west of Junction 14.