How to Jailbreak iPhone 3GS on iOS 4, which is already Jailbroken Using Spirit with Spirit2Pwn
When iOS 4 jailbreak first came out, it was only compatible with iPhone 3GS (old bootrom), iPhone 3G and iPod touch 2G (non-MC models). Things were even more trickier for iPhone 3GS. These 3GS devices were not only required to have old bootrom, but they were also required to have previously been pwned or jailbroken on anything (like blackra1n) other than Spirit.
If your iPhone 3GS was not previously jailbroken with anything other than Spirit, iTunes used to throw an error whenever you tried to restore your phone with custom firmware. Sn0wbreeze 1.6.2 later brought support for these Spirit’ed devices. But in case if you are still getting problems restoring to custom cooked iOS 4 firmware (from PwnageTool or Sn0wbreeze), you can use Spirit2Pwn patch to perfectly restore your iPhone 3GS on iOS 4custom firmware.