iOS 4 is the fourth version of the iOS mobile operating system designed by Apple Inc. It was released on June 21, 2010, and was the first major release to be renamed to simply "iOS", and the first major release to drop support for some devices, as well as the first update that iPod Touch users did not have to pay for.
The iPhone 3G and 2nd generation iPod Touch lack multitasking capabilities and the ability to set a home screen wallpaper, while the iPhone 4, iPhone 3GS, 3rd and 4th generation iPod Touch have them. The iPhone 2G and the 1st generation iPod Touch cannot run iOS 4 and above. More: en.wikipedia.org.