Ultrasn0w 1.2.3 is a good tool to unlock iPhone. In this case to unlock iPhone 4 or 3GS running on iOS 4.3.3, you also can use this tool. First you must jailbreak your iPhone first. You can find the links for guide to in the menu above.
Support Baseband: 01.59.00 / 04.26.08 / 05.11.07 / 05.12.01 / 05.13.04 / 06.15.00
Now I assume that you have done with jailbreaking job that you can follow our guide here to jailbreak iOS 4.3.3 here and you can follow the guide below:
Step 1: Open Cydia, then go to “Manage” then “Add Sources”
Step 2: On the new screen select “Edit” then “Add”. This is very important that you have to enter the exact source : “http://repo666.ultrasn0w.com” and select “Add Sources”.
Step 3: Now tab on “Search” next to Manage to find ultrasn0w 1.2.3. Now you found it in the search list then you can tab on it and tab on “Install” on the top right corner.
Congratulation now, you have successfully unlock your iPhone 4 or 3GS with iOS 4.3.3. Restart the phone and enjoy the new service.
You can find more guide about jailbreaking and unlocking iPhone, iPod and iPad with links in the top menu.