The A4 is a major road in England, also known as the Great West Road. It runs from London to Avonmouth, near Bristol. Historically the road has been the main route from London to the west of England, and has formed the main western artery of London. Much of the route has now been bypassed by the M4 motorway.

Starting at Holborn Circus in the City of London, it runs west into Westminster through Fleet Street, the Strand, Trafalgar Square, Haymarket, Pall Mall, Picadilly Circus, Green Park, Hyde Park Corner and Knightsbridge. The road runs past some of London's most famous buildings and institutions, including the Royal Courts of Justice, London School of Economics, St Martin-in-the-Fields Church, Bush House, Nelson's Column, the National Gallery, Royal Academy of Arts, Ritz Hotel, Harrods, the Victoria and Albert and Natural History Museums and Heathrow Airport. The road is London's main western artery, forking into the old A4, M4 motorway and A30 in the suburbs.

Outside London the road runs through Slough, Maidenhead, Reading, Newbury, Hungerford, Marlborough, Calne, Chippenham, Corsham, Bath and Bristol. In Bristol the road forms an inner city ring road and runs through the Avon Gorge, terminating at the M5 motorway and Avonmouth docks.

The road was formerly classified as a trunk road, but since the 1960s the M4 motorway has relieved the road of long distance and freight journeys. Sections in Bath, Bristol and central London remain designated as trunk road, and traffic is mostly segregated on dual-carriageway on these sections.