ultrasn0w is a program created by the iPhone Dev Team which serves as a Carrier Unlock for jailbroken iPhones. The current supported baseband versions for 3G/3GS for this unlock tool are 4.26.08, 5.11.07, 5.12.01, 5.13.04, and 6.15.00. The only current supported baseband for iPhone 4 GSM is 1.59.00. (The “baseband” version is called the “modem firmware” version in the iPhone Settings app.)