Highway 4 begins at the junction of PH 15 in Dayuan, near Taiwan Taoyuan International Airport.

The highway then continues through Luzhu, and is one of the main roads in Nankan (南崁) area, one of the fastest-growing suburbs in Taoyuan City.

Highway 4 continues as Sanmin Road until it reaches Bade, where it becomes Jieshou Road (介壽路) and crosses Freeway 2.

The road then enters the suburban district of Daxi, where it runs concurrent with PH 3 before splitting near old town Daxi.

The highway continues and ends at the north rim of Shimen Dam in Longtan.