In Short

iOS 4, code-named Apex and originally previewed as iPhone OS 4.0, was introduced by Apple during a special event on April 8, 2010. Tentpole features included limited multitasking for App Store apps, Folders, iBooks for iPhone, Game Center, iAd, FaceTime, and more. iOS 4.1, code-named Baker, shipped on September 8, 2010 and added a FaceTime app for iPod touch, HDR photography, a social music network named Ping (since shut down), and more. iOS 4.2, code-named Jasper, shipped on November 22, 2010. It unified iPhone and iPad software releases, and included AirPlay, AirPrint, and more. iOS 4.3 , code-named Durango, shipped on March 9, 2011, and included features then-specific to the iPad 2 such as AirPlay Mirroring, navigation gestures, FaceTime app for iPad, Photo Booth app for iPad, and more. The last version was iOS 4.3.5, which shipped on July 25, 2011.