Otoyol 4, named Anatolian Motorway and abbreviated as O-4, is a toll motorway connecting the northwestern Marmara Region to the Central Anatolia Region in Turkey. It runs parallel to the D.100 for more than half of its length and then parallels the D750 for most of the eastern half. 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. The O-4 also makes up part of the International E-road network E80 and E89 as well as the AH-1 of the Asian Highway Network.

The Otoyol is the national network of controlled-access highways in Turkey. The term Otoyol translates to motorway while the literal meaning is auto-route. The Otoyol was first opened in 1973.