STEP 1: First, you will have to jailbreak your iPhone 4 or 3GS on iOS 4.3.3. Instructions on how to jailbreak iOS 4.3.3 using Redsn0w 0.9.6RC15 can be found here.
STEP 2: Once you are done with the jailbreak, you can now unlock your iPhone on the supported basebands using Ultrasn0w 1.2.3 from Cydia. To do this:
Touch on â€œEditâ€ and then on â€œAddâ€. You will be prompted to enter a url source as seen in the screenshot below. Type â€œhttp://repo666.ultrasn0w.comâ€ and touch onâ€œAdd Sourceâ€ to add this repo if you havenâ€™t already. Cydia will now automatically update your sources by following a series of automated steps.
After installation is completed. Search for â€œultrasn0w 1.2.3â€ in Cydia and install this application. This app will automatically unlock your iPhone so that you can use it with any carrier.
STEP 3: Now simply restart your iPhone and voila! you should now have a fully unlocked iPhone 4 or 3GS, running iOS 4.3.2 !