iPhone 4 was amongst the oldest devices to receive the iOS 7 update. However, many users complained that the update made their phones buggy and slow. Thankfully, this problem has finally been corrected with iOS 7.1, which not only adds some new features, it also makes the phone respond quicker.
iOS 7 affected iPhone 4 adversely since it was made for applications that were not made for the iPhone 4, such as AirPlay, AirDrop and Siri. iOS 7.1, on the other hand is better optimized and has improved the functioning of basic apps such as Safari, Camera, Calendar and Mail. As you can see from the above chart created by Ars Technica, iOS 7.1 brings up speed for the iPhone 4 for basic applications and booting but is still not as fast as iOS 6.1.3.
“iOS 7.1 also helps with the UI jerkiness that was all over the place in 7.0. It’s easiest to capture the difference in video—actions like launching apps or pulling up the Control Center shade are always visibly jerky in iOS 7.0, but they’re smooth (or at least smoother) in the new update.”
If you are still hanging on to your iPhone 4, iOS 7.1 might be a good bet, atleast better than iOS 7.