Those users having iPhone 3GS will be knowing that its not possible to jailbreak iOS 4.0 on iPhone 3GS with any existing jailbreak tools like Pwnagetool 4.0.1 or Sn0wbreeze 1.6.2 since both the tools are only meant for iPhone 3GS with old bootrom (how to check iPhone 3GS bootrom).
But now you don’t have to disappoint if you have iPhone 3GS with new bootrom since iH8sn0w has come up with guide to jailbreak iOS 4.0 on iPhone 3GS new bootrom though procedure is quite lengthy & has most important requirement of saved SHSH blobs of 3.1.2 firmware for your iPhone 3GS.
If you don’t know what you are doing then you must stay away from this iPhone 3GS 4.0 jailbreak method. This guide is not for regular user.
Follow complete step by step guide to jailbreak iPhone 3GS with New bootrom here.