The O-4 is a major expressway in Turkey as it connects eastern Istanbul and the heavily urbanized northeastern shore of the Sea of Marmara to the nation's capital, Ankara.

Construction of the O-4 began in 1984 and was widely completed in 1992 except for a short gap through the mountains west of Bolu.

[1] After the motorway crosses Necip Fazıl Boulevard, the last toll-free exit in Istanbul (K1), the O-4 becomes a toll-motorway at the Çamlıca toll booths.

Exit K4 is located right on the eastern border of the Pendik district and connects to the Sabiha Gökçen Airport.

Exit K5 (Orhanlı junction) also offers a connection to the Sabiha Gökçen airport as well as the Kurtköy-Pendik connector.

[8] Following the junction, the motorway runs along the heavily industrialized northern shore of the Izmit Bay with exits at Diliskelisi (K8)[9] and Hereke (K9, K10).

The route then heads north and travels around the hilly northern perimeter of the city, passing through two tunnels and a viaduct.