Following one or more steps listed below will definitely help to speed up your iPhone 4.Even though iPhone 4 is one of the fastest iOS device with A4 chip, it tends to get slower after installing few applications. Apple also underclock iPhone 4 CPU in order to improve the battery life and few other reasons.
Credit: iPhone 5 Jailbreak Blog
Apple also underclock iPhone 4 CPU in order to improve the battery life and few other reasons.
Text messages: Delete unwanted old SMS. Leaving as much free space as you can is always a good choice.
Do let us know if the above steps has helped you to speed up your iPhone 4 (above steps are also applicable to other iOS devices). Have we missed something? Let us know your view via comments section.