Westbound The M4 starts six miles west of central London where the A4 merges into the M4 on a raised flyover. (The A4 continues in its own right below the flyover.) Junction 1 (restricted access): A205 South Circular, A406 North Circular, A315 Chiswick, A4. Junction 2: A4 Staines, Hounslow. Heston services. Junction 3: A312 Heathrow Airport Terminal 4 and cargo, Hayes, Harrow, Feltham. Junction 4: M4 spur Heathrow Airport terminals 1, 2 & 3, A408 Uxbridge. Access via spur to Junction 4a and A4 east and west. Junction 4b: M25 Heathrow Airport Terminal 4 and cargo, Gatwick Airport, Watford, Oxford. Junction 5: A4 Colnbrook, Langley, B470 Dachet. Junction 6: A355 Slough central, A332 Windsor. Junction 7: A4 Slough west. Junction 8/9: A404M High Wycombe, Henley, A308M Maidenhead. Junction 10: A329M Reading east, Wokingham, Bracknell. Junction 11: A33 Basingstoke, Reading central and south. Reading services. Junction 12: A4 Theale, Reading west. Junction 13: A34 Newbury, Oxford, Chieveley services. Junction 14: A338 Hungerford, Wantage. Membury services. Junction 15: A346 Marlborough, A419 Swindon, Gloucester. Junction 16: A3102 Swindon west, Wootton Bassett, Calne. Junction 17: A350 Chippenham, A429 Cirencester. Leigh Delamere services. Junction 18: A46 Bath, Stroud. Junction 19: M32 Bristol. Junction 20: M5 The south west, Bristol west, The midlands, Gloucester. Junction 21: M48 Chepstow This is the old M4 Junction 22: M49 Avonmouth. Second Severn Crossing (bridge). Junction 23: No exit Junction 23a: Magor services. B4245 Magor, Caldicot. Junction 24: A455/A48 Newport east, A449 Monmouth. Junction 25: B4596 Caerleon. Junction 25a: A4042 Newport, Cwmbran. Junction 26: A4051 Newport. Junction 27: B4591 High Cross. Junction 28: A48 Newport, A467 Risca, Brynmawr. Junction 29: A48(M) Cardiff east and south. Junction 30: A4232 Cardiff east. There is no Junction 31 Junction 32: A470 Cardiff north, Merthyr Tydfil. Junction 33: Cardiff West services. A4232 Cardiff west, Barry, Penarth, airport. Junction 34: A4119 Llantrisant, Rhondda. Junction 35: A473 Bridgend, Pen-coed. Junction 36: Sarn Park services. A4061 Bridgend, A4063 Maesteg. Junction 37: A4229 Porthcawl, Pyle. Junction 38: A48 Port Talbot. There is no Junction 39 Junction 40: A4107 Port Talbot. Junction 41: A48 Briton Ferry. Junction 42: A483 Swansea and ferry. Junction 43: A465 Neath, Merthyr Tydfil. Junction 44: A48 Swansea east. Junction 45: A4067 Swansea, Pontardawe. Junction 46: B4489 Swansea north. Junction 47: Swansea West services. A483 Swansea west. Junction 48: A4138 Pontarddulais, Llanelli. Junction 49: End of motorway. Pont Abraham services. A48 Carmarthen, Pontarddulais, A483 Llandeilo. Eastbound Start of the motorway. Pont Abraham services. A48 Carmarthen, Pontarddulais, A483 Llandeilo. Junction 48: A4138 Pontarddulais, Llanelli. Junction 47: Swansea West services. A483 Swansea west. Junction 46: B4489 Swansea north. Junction 45: A4067 Swansea, Pontardawe. Junction 44: A48 Swansea east. Junction 43: A465 Neath, Merthyr Tydfil. Junction 42: A483 Swansea and ferry, A48 Briton Ferry. Junction 41: A48 Port Talbot. Junction 40: A4107 Port Talbot. There is no Junction 39 Junction 38: A48 Port Talbot. Junction 37: A4229 Porthcawl, Pyle. Junction 36: Sarn Park services. A4061 Bridgend, A4063 Maesteg. Junction 35: A473 Pen-coed. Junction 34: A4119 Llantrisant, Rhondda. Junction 33: Cardiff West services. A4232 Cardiff west, Barry, Penarth, airport. Junction 32: A470 Cardiff, Merthyr Tydfil. There is no Junction 31 Junction 30: A4232 Cardiff east. Junction 29: No exit. Junction 28: A48 Newport, A467 Risca, Brynmawr. Junction 27: B4591 High Cross. Junction 26: A4051 Newport, Cwmbran, Caerleon. Junction 25a: No exit Junction 25: No exit Junction 24: M50 The Midlands, A48/A455 Newport, A449 Monmouth. Junction 23a: Magor services. B4245 Magor, Caldicot. Junction 23: M48 Chepstow This is the old M4 Second Severn Crossing (bridge). Junction 22: M49 Avonmouth, The south west. Junction 21: No exit Junction 20: M5 The south west, Bristol west, The midlands, Gloucester. Junction 19: M32 Bristol. Junction 18: A46 Bath, Stroud. Leigh Delamere services. Junction 17: A350 Chippenham, A429 Cirencester. Junction 16: A3102 Swindon west, Wootton Bassett, Calne. Junction 15: A346 Marlborough, A419 Swindon east. A420 Oxford. Membury services. Junction 14: A338 Hungerford, Wantage. Unnumbered junction reserved for military use Junction 13: A34 Newbury, Oxford, Chieveley services. Junction 12: A4 Theale, Reading west. Reading services. Junction 11: A33 Basingstoke, Reading central and south. Junction 10: A329M Reading east, Wokingham, Bracknell. Junction 8/9: A404M High Wycombe, Henley, A308M Maidenhead. Junction 7: A4 Slough west. Junction 6: A355 Slough central, A332 Windsor. Junction 5: A4 Colnbrook, Langley, B470 Eton, Dachet. Junction 4b: M25 Heathrow Airport Terminal 4 and cargo, Gatwick Airport, Watford, Oxford. Junction 4: M4 spur to Heathrow Airport terminals 1, 2 & 3, A408 Uxbridge, Hayes. Access via spur to Junction 4a and A4 east and west. Junction 3: A312 to Heathrow Airport Terminal 4 and cargo, Hayes, Harrow, Hounslow. Heston services. Junction 2 (restricted access): A205 South Circular, A406 North Circular, A315 Chiswick, A4. Junction 1: No exit The M4 ends six miles west of central London where it becomes the A4. See also List of motorways in the United Kingdom