I'm planning to jailbreak my iPhone 3G with redsnow to ios 4.0. I currently have 3.1.3 and iTunes 9.1. I looked at the guide to determine what I'm supposed to do, but do I install the official 4.0 from apple first, using iTunes 9.2 and then jailbreak using the Ipsw as per the guide, or can I simply jailbreak from 3.1.3. According to the guide, I upgrade to 4.0 from apple, then jailbreak. However, I just want to be sure.