Jailbreak iPhone 4 and you are liberated! If you are asking
yourself what will I be able to achieve after a jailbreak then you need
not look any further. The following reasons should help you to make
the decision.
• You will be able to install third party applications.
• Add customized ringtones at not additional charge.
• Enjoy Multimedia Applications with live video streaming.
• Download and save YouTube videos.
If after the jailbreak you probably realize that you do not like what
you have done then you can easily restore your iPhone 4 to its factory
settings by using iTunes.
Sources:
Jailbreak IOS 4
Jailbreak IOS 4.1
Jailbreak 4.3.1