iOS 4 was a big update or if not the biggest update in the iPhone timeline. iOS 4 was originally released for the big upgrade on the new iPhone 4 and was supported by iPhone 3GS and iPhone 3G as well. Since then, Apple has released numerous iOS 4.xx updates fixing issues and adding new features.
You might be thinking that most people update to latest iOS like I did, but that’s not the case. According to this graph and statistics conducted by the advertising bureau Apprupt, 95% of iPhone users still have their device running on iOS 4, while iOS 3 runs on 4 percent of iPhone user. There are also some iPhone running iOS 2 as well, but not that many at a mere of 0.17%.