Hacking the iPhone 4 basically means 'jailbreaking' it. This can very easily be done by navigating to '
Jailbreak me' on your iPhone using Safari and follow the instructions on the screen.
The only catch here is that the firmware on your iPhone does not exceed version 4.0.1. (at the moment).
To check what version of the firmware your phone has, go to:
Settings --> General --> About
the firmware version should be displayed under 'Version'.
A new jailbreak tool has been released call
Greenpois0n. Have a look at their website to see how it works. This tool will work with all iOS devices upto version 4.1.
July 25th 2011
Jailbreaking if fairly easy at the moment, just browser to www.jailbreakme.com on your device, and follow the instructions on the screen.