Otoyol 4

O-4
O-4
Anadolu Otoyolu
Route information
	Part of E80 E89
	Part of AH1
Length	372 km (231 mi)
Existed	1984–present
Major junctions
West end	O-1, in Istanbul
	O-2 in Ataşehir; O-7 in Sultanbeyli; D.100 in Gebze; O-5 in Gebze; D.605 in Izmit; D.650 in Arifiye;
East end	O-20 in Ankara
Location
Country	Turkey
Regions	Marmara, Black Sea, Central Anatolia
Provinces	Istanbul, Kocaeli, Sakarya, Düzce, Bolu, Ankara
Major cities	Istanbul, Izmit, Bolu, Ankara
Highway system
	Highways in Turkey; Motorways List; ; State Highways List; ;

Otoyol 4 (English: Motorway 4), named Anatolian Motorway (Turkish: Anadolu Otoyolu) 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 O-4 is the longest completed motorway in Turkey spanning 379 km (235 mi), just 14 km (8.7 mi) longer than the O-52. The partially-opened O-5 is expected to surpass the length of the O-4, when it is completed in 2019, with a total length of 407 km (253 mi). but nowadays the longest motorway in Turkey is Otoyol 21 with the opening Ankara-Niğde section. 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. This gap was connected in 2006, with the opening of the Mount Bolu Tunnel.