IOS 4

iOS 4 (codenamed "Apex") was the 4th major release of iOS designed and developed by Apple for the iPhone, iPod touch and iPad, being the successor to iPhone OS 3

New features
iOS 4 was developed for the iPhone 3G, 2nd-generation iPod touch, and 1st-generation iPad and later. The update was free and included a multitude of new features. The features included, but were not limited to:


 * Multitasking
 * Folder organization
 * Multiple mail boxes
 * iBooks
 * Support for iMovie (available in the App Store)
 * Creation of music playlists
 * 5x digital zoom for camera (iPhone 4 and iPod touch (4th gen) only)
 * LED flash (iPhone 4 only)
 * Tap-focus for video
 * Picture identification
 * Home screen wallpaper
 * Ability to "gift" apps
 * Spell checking
 * Bluetooth keypad
 * Ability to rent movies
 * Game Center
 * FaceTime (iPhone 4 and iPod touch (4th gen) only)
 * Personal hotspot (iPhone 4 only)

Support

 * The 1st and 2nd-generation iPads, 3rd and 4th-generation iPod touches, iPhone 3GS, and iPhone 4 were fully supported.
 * The iPhone 3G and 2nd-generation iPod touch had limited support, which was discontinued after version 4.2.1.
 * The 1st-generation iPhone and iPod touch were no longer supported by any version of iOS 4.

Limitations on older models
Some features were disabled on older models due to hardware limitations, eventually leading Apple to stop supporting them entirely after version 4.2.1:
 * Compass (unavailable for iPhone 3G and 2nd-generation iPod touch)
 * Game Center (unavailable for iPhone 3G)
 * Home screen wallpaper (unavailable for iPhone 3G and 2nd-generation iPod touch)
 * Multitasking (unavailable for iPhone 3G and 2nd-generation iPod touch)
 * Nike + iPod (unavailable for iPhone 3G)
 * Voice Control (unavailable for iPhone 3G and 2nd-generation iPod touch)

End of support
With the announcement of iOS 5 on June 6 at the 2011 Worldwide Developers Conference, owners of the iPhone 3GS, iPhone 4, iPod touch (4th generation), and iPad 2 were concerned about the level of support that their devices would continue to receive (see List of unsupported firmware). The final updates were released on July 25, 2011.

iOS 4 continued to be supported until October 12, 2011, when it was superseded by the release of iOS 5.0, which introduced iCloud services.

Trivia

 * This was the second major release to drop support for some devices.
 * This is the first iOS to introduce the home screen wallpaper.
 * This is the first iOS to to use multitasking.
 * This is the last iOS to feature the iPod app.
 * This is the last iOS to have the Mona Lisa wallpaper on Settings.