For most of its length SH4 is a two-lane single carriageway, with at-grade intersections and property accesses, both in rural and urban areas.

[1] The highway commences from a junction on SH 3 11 km south of Te Kūiti.

It heads south-southeast, climbing initially, then descending along the winding valleys of the Ōhura and Ongarue Rivers, two tributaries of the Whanganui.

The Ongarue joins the Whanganui close to Taumarunui, and here the highway turns east briefly, ascending the Whanganui's valley before turning south to skirt the North Island Volcanic Plateau.

The highway continues to descend the Mangawhero's valley to within 40 km of its mouth, then heads west across rolling hill country to the valley of the Whanganui River, which it follows for the last 15 km of its journey into Whanganui.