Line S4 of Suzhou Rail Transit (Chinese: 苏州轨道交通S4线) will be a north–south rapid transit express line. The line will run through Zhangjiagang, Changshu, Xiangcheng, Gusu, Wuzhong, and Wujiang. The line will serve as a rapid connection between the main city of Suzhou and the county-level city of Changshu, and to a lesser extent, Zhangjiagang.^[1]

There is no planned construction date, but construction is expected to start before 2035.