This software also allows you to customize a jailbroken iPhone iOS 4.2.1 while preserving your version baseband model for the iPhone.
Download Sn0wBreeze 2.2
You can download Sn0wbreeze 2.2 for Windows from the link below:
Download Sn0wBreeze 2.2 [Mirror 1] [Mirror 2]
Download iOS 4.2.1 for iPhone, iPad, iPod touch
Download .NET Framework (required to run Sn0wbreeze)
Mac users can follow our PwnageTool 4.2 step by step guide posted here.
Once you are done with the jailbreak with the iPhone, you can unlock your iPhone 4, 3GS and 3G on the old baseband using Ultrasn0w 1.2.