Okay so to start off I have an iPhone 3GS with version 4.0 (8A293) with modem firmware of 05.13.04. I believe this is all in check with being able to jailbreak and unlock the phone so I tried.
First, I jailbroke the phone with limerain, and then ran Cydia and tried to instal ultrasn0w. But I get an error (cannot comply), where it has a conflict: 'iOS Firmware >=3.0'. As far as I know I can still use ultrasn0w to unlock the phone.
A little advice would be greatly appreciated, thanks!