The M4 Motorway is a 55-kilometre (34 mi) series of partially tolled dual carriageway motorways in Sydney designated as route M4. The M4 designation is part of the wider A4 and M4 route designation, the M4 runs parallel and/or below ground to Great Western Highway, Parramatta Road and City West Link, which are part of route A44.

The M4 Motorway comprises two connected parts:

Western Motorway commences at the transition point from Great Western Highway in Glenbrook and heads in a westerly direction as a four-lane, dual-carriageway road, crossing the Nepean River and widening to six lanes at the interchange with Mulgoa Road in Regentville, then widening again to eight lanes at the interchange with Roper and Erskine Park Roads in Erskine Park. The motorway passes under Wallgrove Road and Westlink M7 at the four-level Light Horse Interchange, continuing east through Prospect and Merrylands. At the interchange with Church Street and Woodville Road in Parramatta, tolling begins and the motorway splits into three carriageways (one four lanes eastbound, two dual-lane westbound), before merging back into two carriageways just west of the Duck River in Clyde. 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 motorway eventually terminates just east of the Rozelle Interchange, returning to the surface at Rozelle to connect to the Western Distributor.

The M4 cycleway runs parallel to the M4 Motorway between Sydney Olympic Park and South Wentworthville.

The first main road west from Sydney was Great Western Highway, shown above in orange. The County of Cumberland planning scheme provided for a modified route west, much of which was later built as M4 Western motorway. With the opening of the M4 East Tunnel and Rozelle Interchange in 2019 and 2023 respectively, the M4 today extends as far east as the Anzac Bridge.

The Western Freeway was originally constructed in several stages between the late 1960s and the mid-1980s: the first 5 km-long section between Emu Plains and Regentville (including a new single carriageway bridge over the Nepean River, duplicated in 1986) opened in October 1971; the second 14.5 km-long section between Regentville and Eastern Creek opened in December 1972; the third 4 km-long section between Eastern Creek and Prospect opened in April 1974. 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. Separate sections of Western Freeway, from Great Western Highway in Mays Hill to Church Street and Woodville Road in Parramatta, and from Auburn to Great Western Highway at Strathfield, were opened on 16 December 1982 by Premier Wran. In September 1984, a 2.1 km section of freeway opened to traffic, extending the freeway west from Auburn to James Ruse Drive in Granville. The 1.8 km Granville Viaduct project opened to traffic in April 1986, the elevated section finally connecting the Granville end to the next section at Parramatta and allowing both Granville and Parramatta to be properly bypassed. As part of this project, ramps to and from the north side of James Ruse Drive were opened to traffic in August 1987.

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. In December 1989 work to construct this stage began as a Build-Own-Operate-Transfer project. In return for funding construction StateWide Roads, the consortium awarded to build the stage, was given permission to toll the section between James Ruse Drive and Silverwater Road as traffic volumes on this section were significantly higher than between Mays Hill-Prospect and would allow a shorter toll period with lower tolls. The consortium would also widen the section between James Ruse Drive and the newly constructed Homebush Bay Drive to six lanes. The Mays Hill to Prospect section opened in May 1992 and an initial of a $1.50 toll was implemented; as a consequence of the toll, the freeway also changed names to Western Motorway. The concession held by StateWide Roads ended on 15 February 2010, with operation of the motorway returned to the Roads & Traffic Authority and the toll removed. At the time, StateWide Roads was owned by Transurban (50.6%) and Utilities Trust of Australia (21.5%).

The motorway is mostly three or four lanes wide in either direction, and carries constant heavy traffic during daylight hours, seven days a week. Built as a four-lane motorway, it was widened to six lanes during 1998 to 2000, but this did little to ease the congestion.^{[citation needed]}