The line runs for 42km from Longdaobang in the north to Tongli in the south, with a 10.8km branch from Hongzhuang to Muli, which will ultimately become part of the future Line 7.
The Yuan 36bn ($US 5.2bn) line adds 37 stations to the network, including interchanges with metro Line 1 and the main line network at Suzhou Railway Station.
Peak services operate at six-and-a-half minute headways on the main line, with trains running every seven-and-a-half minutes on the branch to Muli.
Services are operated by a fleet of 40 six-car type B trains supplied by CRRC Nanjing Puzhen.
The completion of Line 4 takes the total length of the Suzhou metro network to 121km with 97 stations.
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