SR 4 begins at an intersection with US 101 at Johnston's Landing, located on the Naselle River in rural Pacific County.

SR 4 continues through the northern part of Longview and passes the campus of the Lower Columbia College before traveling onto Cowlitz Way.

[6] SR 4 turns east onto Allen Street and continues to its eastern terminus, a diamond interchange with I-5 north of the Three Rivers Mall.

In 2011, WSDOT calculated that the busiest section of SR 4 was the intersection with Catlin Street in Kelso west of the Cowlitz Way Bridge, serving 34,000 vehicles daily, while the least busy section was between Rosburg and Cathlamet in rural Wahkiakum County, serving less than 1,000 vehicles per day.

[4][26][27] US 830 was decommissioned in 1968 and was replaced with SR 4;[2][28] however, the highway was proposed as a route for US 12 before it was extended west via White Pass to Aberdeen.