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

The iOS 4 round of updates had finished, it started with iOS 4.0 and concluded with 4.3.5 (iPad 2 only) and 4.3.4 (all other devices (excluding iPhone 3G and iPod Touch (2nd generation), it was succeeded by iOS 5 on October 12, 2011.
 * Multitasking
 * Folder Organisation
 * Multiple Mail Boxes
 * iBooks
 * support for iMovie (available in the App Store)
 * Creation of Playlists
 * 5x Digital Zoom for Camera (iPhone 4 and iPod Touch 4G)
 * LED flash (iPhone 4 only)
 * Tap-Focus for video
 * Picture Identification
 * Homescreen Wallpaper
 * Ability to 'Gift' Apps
 * Spell Check
 * Bluetooth Keypad
 * Ability to rent movies
 * Game Center
 * FaceTime (iPod Touch (4th generation) and iPhone 4 only)
 * Personal hotspot (iPhone 4 only)

Limitations on older models
Some features were unavailable on the iPhone 3G and iPod touch (2nd generation):
 * Compass (disabled)
 * Voice Control (disabled)
 * Nike + iPod (Disabled only on iPhone 3G)
 * Game Center (Disabled only on iPhone 3G)

Support

 * The 1st generation iPhone and iPod touch were not supported in this release.
 * The iPhone 3G and 2nd generation iPod touch had limited support.
 * All the iPads, 3rd and 4th generation iPod touch, iPhone 3GS, and iPhone 4 were fully supported

End of support
iOS 4's support was ending as of October 4, 2011. iPhone 3GS, iPhone 4, iPod touch (4th generation), iPad 2 Wi-Fi and iPad 2 Wi-Fi + 3G were concerned that iOS 5 was going to come and be an update. Soon, iOS 4 was completely discontinued. Its features were moved to iOS 5. (See List of unsupported firmware)

Discontinuation
iOS 4 was still supported for a limited time from June 6 to October 10, 2011. Apple made it unsupported to release iOS 5 (iOS's 5th and next level), having released 5.0 on October 12, 2011. All services were moved to iOS 5 and iCloud.

Trivia

 * Initial release on iPhone 4 and iPod touch (4th generation).
 * This was the 2nd release to drop support for some devices.
 * Recent version was un-available for the CDMA version of the iPhone 4.
 * This is the last iOS to feature iPod and Voice Control.
 * This is the last iOS to have the Mona Lisa wallpaper on Settings.
 * This is the first iOS to introduce the home screen wallpaper.
 * This is the first iOS to to use multitasking.
 * Neither multitasking nor the home screen wallpaper were supported on the iPhone 3G or second-generation iPod touch due to hardware limitations.