Chiswick - Central London
The short section of the A4 from the Chiswick Roundabout to the end of the M4 is essentially just a slip road, taking traffic over the top of the A3000 and forming an additional lane that joins the two lanes of traffic coming off the Chiswick flyover. The resulting three-lane dual carriageway crosses the Piccadilly Line and then skirts Wolseley Gardens on the right, continuing into a built-up area on both sides. The name "Great West Road" is often used for the whole of this section, but much of it runs along the route of pre-existing roads, this part becoming Cedars Road.
Several of the former junctions with side streets have now been closed, but it's still possible to make a left turn into Sutton Lane North. This is a deliberate arrangement to allow traffic to circumvent the forbidden right turn at the next junction, as traffic for Grove Park runs round three sides of a rectangle. The signal-controlled junction with Sutton Court Road allows left turns only.
The A4 continues along Ellesmere Road and Hogarth Lane, passing Hogarth's House on the right, to the Hogarth Roundabout, a well-known west London traffic landmark. This is a large signal-controlled roundabout where the A316, carrying traffic from south-west London and the M3, crosses the route of the A4 and heads northwards into Chiswick. It is notable for the single-lane flyover carrying northbound A316 traffic over the roundabout - apparently a temporary structure that has lasted for several decades. There's no way for A4 traffic to avoid the roundabout, though, and the three lanes of traffic continue through the junction and out past Fuller's Brewery on the right, joined by right-turning A316 traffic that filters in from the flyover on the left.
The next stretch passes the Homefield Recreation Ground on the left and runs close to the River Thames. Here it's necessary to get in lane, as traffic in the left-hand lane is directed into Hammersmith, while the other two lanes continue over the Hammersmith Flyover, a substantial elevated section that avoids the whole of the Hammersmith one-way system. At the end of the flyover the third lane is restored and the A4 continues along Talgarth Road.
The pace of traffic is noticeably slower now. After passing the London Academy of Music and Dramatic Art on the right, the road passes through a set of signals near Barons Court station and another near West Kensington. Renamed to West Cromwell Road, it passes over the top of the West London railway line near Lillie Bridge depot. The three lanes are quite narrow here and this point was marked as an "accident black spot" for many years.
The next signal-controlled junction is with Warwick Road, which forms the northbound half of the A3220, not far from Earl's Court Exhibition Centre. Traffic can turn left here towards Shepherd's Bush and the A40. Unsurprisingly, it's not long before we encounter southbound A3220 traffic coming along Earl's Court Road towards the Embankment, at another signal-controlled junction. A lot of traffic leaves the A4 here and there are three dedicated right-turn lanes.
The A4 continues along Cromwell Road proper, passing the Cromwell Hospital on the left, and the dual carriageway soon turns into a wide single carriageway, briefly becoming dual again near Gloucester Road station, but then back to single for the rest of this stretch. We're now moving into south Kensington and museum-land, and after crossing Queen's Gate we see the grounds of the Science Museum on the left. Crossing Exhibition Road takes us into Cromwell Gardens and past the Victoria and Albert Museum.
Moving into Thurloe Place, we pass the entrance to the famous Brompton Oratory school and then pass into Brompton Road, which swings north-eastwards to a junction with the A315 near Knightsbridge station. The layout of this junction makes it clear that this is an important stage in the route - the A315 is the original coaching route, which would presumably have formed part of the A4 if the Great West Road had not been built. The road does a slight dog-leg to turn right into Knightsbridge, passing the French Embassy on the left and then skirting Hyde Park.
This point marks the start of the Piccadilly underpass, which runs under Hyde Park Corner. The A4 once again becomes a dual carriageway, with the two carriageways running in separate bores. Because this is the junction with the inner ring road, the A4 loses primary status here and the number no longer appears on signs. It emerges on the eastern side of Hyde Park Corner where it joins Piccadilly, running along the north-eastern side of Green Park, and past Green Park underground station.
Then something odd happens. According to some maps, the A4 splits into two branches here - one turning right into St James's Street and then left into Pall Mall, and the other going straight ahead along Piccadilly to Piccadilly Circus and then turning right down the Haymarket. (Haymarket is southbound only - the corresponding northbound route is along Lower Regent Street.) The branches rejoin at Cockspur Street, which leads into the roundabout on the southern side of Trafalgar Square - eastbound traffic would once have proceeded along the northern side of the square past the National Gallery, but this is now closed to traffic.
A4 traffic takes the first exit from the roundabout, turns left into St Martin's Place and then right into Duncannon Street before joining the Strand, which carries traffic in both directions as far as the junction with Lancaster Place (leading to Waterloo Bridge). Then eastbound traffic continues around the Aldwych, which forms a roughly semi-circular diversion from the Strand, which carries the westbound traffic. The two directions of traffic join again as we head into Fleet Street, with the Royal Courts of Justice on the left.
Beyond here the A4 once continued along Fleet Street, Ludgate Hill, St Paul's Churchyard and Cannon Street to end at the Monument. However, there are no longer classified roads in the City of London, and so the A4 takes a left turn at Fetter Lane, then running into New Fetter Lane and terminating at Holborn Circus, where it joins the A40. The purists might object to a single-digit road terminating on a double-digit one, but who ever said road numbering was logical?