Seoul Subway Line 4 of the Seoul Metropolitan Subway is a long line crossing from the southwest to the northeast across the Seoul National Capital Area. The central section in Seoul City is operated by Seoul Metro with some trains offering through service to Korail's Ansan and Gwacheon Lines. The southern terminus (Oido) is in Jeongwang 4-dong, Siheung City, and the northern terminus (Jinjeop) is in Jinjeop-eup, Namyangju-si, Gyeonggi-do; the newly built northern section is correspondingly owned by a company owned by Namyangju-si, with operations contracted to Seoul Metro. In 2022, the Seoul Metro operated section had an annual ridership of 219,587,000 or about 601,608 passengers per day.

Seoul Transportation Corporation, branded as Seoul Metro, is a municipal-owned corporation owned by Seoul Metropolitan Government, and one of the two major operators of Seoul Metropolitan Subway with Korail.