Apple released the iOS 4.0 for majority of its iDevices including iPhone 4, 3GS, 3G and iPod Touch 2nd & 3rd Generation. The update provides a plethora of new features and functionality. The iPhone Dev team has updated Redsn0w to 0.9.5 to jailbreak all of the devices running the iPhone OS 4.0. Below is a step-by-step guide to jailbreak your iDevice.
In order to jailbreak, make sure your device is already updated to iOS 4.0. If you’re device is unlocked, don’t upgrade to the new version as it will update your baseband and remove the unlock.
1) Save the iOS 4.0 ipsw file in your hard drive.
3) Run the installer and provide it the link to the .ipsw update file.
4) Turn OFF your iDevice and plug it to the computer.
5) Press and POWER and HOME button simultaneously of your device for a few seconds to boot into recovery mode.
6 ) Now press the ‘NEXT’ button and wait till it jailbreaks the device. Check for Cydia in your applications after the device restarts to validate the jailbreak.
If you running iOS 3.1.3, follow this guide to untethered jailbreak your iDevice.