Table of versions: iOS 3.x – portable iOS devices
iOS 4.x: fourth major OS release
iOS 4 was made available to the public for the iPhone and iPod touch on June 21, 2010. This is the first major iOS release to drop support for some devices. It is also the first major iOS release that iPod touch users do not have to pay for.
The iPhone 3G and iPod touch 2nd Gen. have limited features, while the iPhone 4, iPhone 3GS, iPod touch 3rd Gen. & iPod touch 4th Gen. have all features enabled. The iPhone and iPod touch 1st Gen. cannot run iOS 4.0 and above.
iPad compatibility was added with the release of iOS 4.2.1 on November 22, 2010.
Table of versions: iOS 4.x – portable iOS devices
iOS 5.x: fifth major OS release
iOS 5 was previewed to the public on June 6, 2011. This is also the second major iOS update to drop support for older devices, in this case; the iPhone 3G, and iPod touch 2nd Generation (MB & MC models). It was released for iPhone 3GS, iPhone 4 (GSM and CDMA), iPhone 4S, iPod touch (3rd and 4th generation), iPad, and iPad 2 on October 12, 2011 at 10am Pacific Time (5pm GMT).