Sn0wbreeze 1.6.2 has just been released. This new update brings fixes for YouTube, iBooks and iTunes Error 2 which has been problematic for most users after iOS 4 jailbreak.
Apart from this, this new version also brings support old bootrom Spirit jailbreak tool users who can now jailbreak their device on iOS 4 with Sn0wbreeze.
Instructions to jailbreak iPhone 3GS (old bootrom), iPhone 3G and iPod touch 2G (non-MC model) on iOS 4 using Sn0wbreeze 1.6.2 as usual are exactly similar to the last release.
Please read all the instructions posted here carefully before upgrading and jailbreaking your phone on iOS 4.
Once you have jailbroken your phone, you can unlock it using ultrasn0w 0.93 (on any baseband), guide for which is posted here.
Download Sn0wbreeze V1.6.2 for 4.0 [Windows]