Line 4 of the Guangzhou Metro is a south-north line on the system that runs between Nansha Passenger Port and Huangcun stations, spanning a total of 60.03 km (37.30 mi) with 24 stations. The sections of the line from Xinzao to Huangcun, Feishajiao to Nansha Passenger Port is underground, while that Jinzhou to Guanqiao is above ground. Line 4's colour is dark green. Like line 5 and 6, trains runs with linear motor technology.

Line 21 of the Guangzhou Metro is a rapid transit rail line in Guangzhou.