- Junction 1: London, A4, A406, A205.
- Junction 2: Brentford, A4.
- Heston service area.
- Junction 3: Hounslow, A312.
- Junction 4: West Drayton, Heathrow Airport. Junction 4A: Heathrow Airport, A4.
- Junction 4B: M25.
- Junction 5: Langley, A4.
- Junction 6: Slough, Eton, Windsor.
By October 2007, any hopes of junction 31 finally being built were dashed when Cardiff Council denied the planning application and renewed plans for the Thornhill interchange were abandoned.
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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.
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The M4 motorway (junction 24 (Coldra) to junction 28 (Tredegar Park), Newport) (temporary speed restrictions) Order 2021 | GOV.
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Junction 47, A48/A483 Interchange, Penllergaer Interchange, M4 Motorway.
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Junction 8/9 near Maidenhead, Berkshire, and High Wycombe, Buckinghamshire is the only one in the UK with dual numbers.
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Junction 36: Bridgend, Sarn Park service area. Junction 37: Pyle, Porthcawl. Junction 38: Margam Park.
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Pincents Kiln Industrial Park is off Bath Road when travelling from Reading, near to junction 12 of the M4 motorway. From the motorway junction take the A4 Reading turn off and take the first left turn into the Pincents Kiln Industrial Park. The road is arrow marked IKEA.
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