The motorway passes under Wallgrove Road and Westlink M7 at the four-level Light Horse Interchange, continuing east through Prospect and Merrylands.

East of the interchange with Homebush Bay and Centenary Drives, the original four-lane alignment of Western Motorway branches off to run along the surface to its old terminus with Great Western Highway outside Concord, and the newer six-lane M4 East alignment enters tunnels and continues eastwards underground.

The County of Cumberland planning scheme provided for a modified route west, much of which was later built as M4 Western Motorway.

[15] On the basis of a pre-election promise made by the NSW Premier Neville Wran in 1976, all land reserved for the expressway between Pyrmont and the (then) eastern termination point at Strathfield was sold off to property developers or declassified as a freeway corridor in 1977 by the State Government.

A lack of funding resulted in the Wran Labor government halting plans to construct the final stage of the freeway between Mays Hill and Prospect in 1985.

[20] The motorway is mostly three or four lanes wide in either direction, and carries constant heavy traffic during daylight hours, seven days a week.

[citation needed] Originally planned in the mid-1950s to start in the Sydney central business district,[21] the eastern section was built only as far west as Pyrmont, as part of the North West Expressway (or F3), a freeway that would connect the Sydney and Newcastle central business districts; this section is now part of the Western Distributor.

[26] In 2013 the state government announced the intention to implement a 'Managed Motorway' scheme on the M4 over the coming years to improve traffic flow.

Mechanisms to be used include improved Variable Message Signs, Ramp metering signals, dynamic speed and incident management, and an upgrade of the Emergency Telephone System.

Construction commenced in March 2015 and in November 2015, it was announced that toll points would be reinstated on this section from 2017 to cover costs of the WestConnex project.

One plan in the 1990s involved extending the M4 eastwards by approximately 5 kilometres (3.1 mi) so that it would subsequently end in Ashfield and be continuous with the City West Link.

In the month before the 2011 state election the NRMA released a report in which it recommended building a tunnel to connect the end of the M4 at Concord and the start of the City West Link, relieving Parramatta Road of enough traffic to convert it into two lanes for slower-moving local traffic, two lanes of light rail and a cycleway.

[33] In October 2012 the NSW government announced their commitment to deliver the WestConnex project, involving widening the existing M4 motorway as well as extending it east with a tunnel from North Strathfield to Taverners Hill.

[34] In June 2015 the tender to design and build the WestConnex M4 Tunnel was awarded to Leighton Contractors, Samsung and John Holland.

This section aimed to reduce travel times between Western Sydney and Port Botany while removing heavy vehicles from surface streets in the Inner West.