Illinois Route 4 is a long state road that runs south from the Interstate 55 business loop around the state capital of Springfield, south to Illinois Route 13 just north of Murphysboro. This is a distance of 170.44 miles (274.30 km).

Illinois 4 starts at Illinois 13 and Illinois 127 at a point about eight miles (13 km) north of Murphysboro. It zigzags through small southern towns such as Steeleville, Sparta, and Marissa, before straightening out near Mascoutah. Illinois 4 is an important road in St. Clair and Madison Counties since it connects many towns on the outskirts of the wider St. Louis metropolitan area (Greater St. Louis; specifically, the Illinois portion, the Metro-East Area). From Carlinville to Springfield, the route is also important since it connects many good-sized towns in Macoupin and Sangamon Counties. Illinois 4 passes through Chatham, a fast-growing city outside of Springfield, before reaching the state's capitol.

Illinois 4 in Springfield is known as Veterans Parkway. It completes the western loop around the city of Springfield, and is a divided four-lane highway that serves Springfield commercial areas such as White Oaks Mall.

It is also worth noting that the northbound lane of IL 4 is in Washington County for less than one mile (1.6 km), on its southwest corner near the unincorporated town of Clarmin where it runs concurrently with IL 13.

Illinois State Bond Issue Route 4 was the first numbered through route between Chicago and St. Louis, as shown on the 1924 Illinois Road Map. As such it was the forerunner of more famous routes US 66 and Interstate 55.

In 1926, a new alignment for Route 4 was opened between Joliet and Lyons, on the north side of the Des Plaines River. The old alignment on the south side of the river through Lemont was renamed Illinois Route 4A and then renamed again in 1967 as Illinois Route 171.

When US 66 was first designated in 1926, it overlapped IL 4 for its entire length, however the section from just south of Staunton to Springfield was originally shown only as "Temporary US 66" while the permanent routing of US 66 is shown as proposed or under construction on a more eastern route away from IL 4 through Litchfield. The new route of US 66 was completed as SBI 16 and SBI 126 in 1930, and the US 66 designation was then removed from IL 4 between Staunton and Springfield. Illinois Route 4 and US Route 66 remained as cosigned routes between the Mississippi River and Staunton, and between Springfield and Chicago until 1935, when the IL 4 designation was dropped from portions where it overlapped US 66, leaving only the portion from Staunton to Springfield as IL 4. This left IL 4A as an orphan alternate route of IL 4 from Joliet to Lyons until it was renumbered as IL 171 in 1967.

The section of modern IL 4 from Staunton to its southern end near Murphysboro was originally IL 43. In 1964, IL 4 was extended on this highway and the number IL 43 was eventually reused in the Chicago area.

A bypassed portion of old route 4 north of Auburn is listed in the National Register of Historic Places as "Illinois Route 4-North of Auburn". It was added in 1998 as structure #98000979 and consists of two c.1920 bridges over Little Panther Creek and portions of Curran and Snell roads. One section is a c.1932 1.53 mile long brick road and the other is a c.1921 Portland cement road 16 feet (4.9 m) wide and 1,277 ft (389 m) long.