iPhone Multitouch Issues

Regardless of what version of iOS I have installed on my iPhone 4, the multitouch on my iPhone 4 is not working like it should. I believe that the iPhone can support multiple finger multitouch in games, apps, etc. However, in games like Fruit Ninja, I can at most use two fingers simultaneously (but it lags). Three or more fingers does not work if I put all fingers at once (3+ fingers will only work if I put one finger down on the screen at a time). In the game Tap Tap Revenge 3, hitting notes is impossible. While before I could hit them with 100% accuracy, the multitouch lags to the point that even though I am hitting the notes, the game does not detect them.

Multitouch should work on my iPhone 4, but it doesn't. Everything I mentioned (3+ finger in Fruit Ninja, Tap Tap) does work fine on another iPhone 3GS without any problems. Neither the iPhone 4 nor the iPhone 3GS are jailbroken. How do I fix this?