It entered into operation on 28 September 2009,[5] and runs from north to south, parallel and to the west of Line 5, through Haidian, Xicheng, and Fengtai Districts in the western half of the city.

In the north, Line 4 begins in Anheqiao, just beyond the Summer Palace, and heads south past the Old Summer Palace, through the university district and Zhongguancun, Beijing's high-tech silicon village, before turning east at the National Library of China and passing the Beijing Zoo en route to Xizhimen.

After entering the 2nd Ring Road at Xizhimen, Line 4 resumes southwards at Xinjiekou and traverses the old city through Xisi, Xidan, Xuanwumen, Caishikou, and Taoranting Park.

It passes the city's high-speed rail link at the Beijing South railway station before reaching the terminus at Gongyixiqiao.

It was planned to run from the Summer Palace, east towards Xizhimen, southeast to Zhongshan Park, terminating at the Beijing Stadium, which near today's Tiantandongmen Station.

[18] On 6 March, four subway trains begin testing while Hong Kong's then Chief Executive, Donald Tsang, visited the project.

[22] The planned stations have been identified as Baiwangshan (百旺山), Xibeiwang (西北旺), Aerospace City West (航天城西), and Yongfeng (永丰).

[26] The JV is responsible for 30% of the investment capital to build Line 4, mainly to finance the purchase of electrical and mechanical equipment, while the Beijing Municipal Government provided the remaining 70%, to cover civil engineering, station, and track work costs.

[27] The PPP JV model was designed to introduce private capital as well as advanced metro management methods to the growing Beijing Subway.