This is the Supported basebands:
01.59.00 / 04.26.08 / 05.11.07 / 05.12.01 / 05.13.04 / 06.15.00
Notes: you must preserve your iPhone 4 baseband 01.59.00 to prevent upgrading to higher baseband in order to unlock on iOS 5 click here

How to Unlock iPhone 4, iPhone 3GS on iOS 5 using Ultrasnow 1.2.4 ?

STEP 1: first jailbreak your iPhone 4 or iPhone 3GS using Redns0w with the guide posted here.
STEP 2: Once jailbroken, head to Cydia -> Manage -> Sources.


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STEP 3: Now tap on Edit -> Add then write down the following
repo: ultrasn0w.com
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STEP 4: After adding the repo, search in Cydia for Ultrans0w and you’ll tap on it then hit install.

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After a respiring and a tethered boot, you’ll get your iPhone 4 and iPhone 3GS unlocked.


GUIDE HOW TO INSTALL UltraSn0w

  1. Run Cydia or Icy
  2. Please add the repo repo666.ultrasn0w.com to Cydia or Icy. That last “o” is actually the number zero “0”! If you use the letter “o” you’ll get an error.
  3. Search for ‘ultrasn0w’ in cydia or icy and install ultrasn0w
  4. Reboot your iDevice
  5. T-Mobile USA users should disable 3G before using ultrasn0w
  6. Enjoy