The user experience provided by my iPhone 4 was very good until the gotofail bug was dicovered and then I had no choice but upgrading to iOS 7. Why? Well, because somebody thought that it might have been a great idea to provide the user with a lag-ish user experience.
I know, I know… there are lots of tutorials out there showing how you can tune-up the iOS 7 by disabling animations, disable background app refresh, disable iCloud Documents (Whoa?!?! :O ) and many other things such as “let’s recalibrate the home button because it’s freaking slow”. Apple, seriously? I mean, I know that iphone 5, 5s are really great, but why did you give iOS 6.1.6 only to the 3GS users and forgot about those who still use iPhone 4?
Here’s a vid that says a lot:
Why did Apple destroy our iPhone 4 experience?
UPDATE: I have risked and upgraded to iOS 7.1 after reading several articles about how it behaves on iPhone 4. The iTunes said that iOS 7.1 fixes the performance issues for iPhone 4, and IT REALLY DOES. So, I recommend to upgrade.