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, Caln].
• 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.

• 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.