Once Apple released the iOS 5 For public the iPhone Dev Team was too quick to release the unlock tool for the iOS 5 for iPhone 4,iPhone 3Gs and 3G using Ultrasn0w as Ultrasn0w 1.2.4 has been out but for those iPhone 4 which have updated basebands the basebands are

The basebands which are unsupported from ultrasn0w 1.2.4 are:

02.10.04 / 03.10.01 / 04.10.01 / 05.14.02 / 05.15.04 / 05.16.02

so to all the iPhones which has the mentioned baseband then you have no other option rather than to unlock expect gevey Sim which will unlock all these basebands running on iOS on all iPhones  so if you need gevey SIM to unlock it you don’t even need to jailbreak iOS using any jailbreak tools

How to unlock iPhone 4 baseband 02.10.04 ,03.10.01, 04.10.01, 05.14.02 ,05.15.04, 05.16.02 using Gevey Sim ,Ultrasn0w 1.2.4 won’t unlock these basebands.

If you wish you jailbreak iOS 5 then you can do it via Redsn0w 0.9.9 b5 which will also get you the custom Firmware from here.

what are required to unlock iPhone 4 using GEVEY SIM,

Get iOS 5 for Public From Here.

iTunes 10.5 from Here

Follow steps to unlock iPhone 4S using gevey Sim

STEP 1: Connect your iPhone 4 into you computer then open Redsn0w 0.9.9b5.

STEP 2: Now select Extras then “Custom IPSW” option so that you can create your pre-jailbroken custom firmware.

STEP 3: Redsn0w will start cooking your custom IPSW and you’ll get it in few minutes.

STEP 4: Once you get your firmware, select ‘Pwned DFU’ from RedSn0w.

STEP 5: Follow the on-screen instructions to get your iPhone in DFU mode.

STEP 6: Once it’s in DFU mode, close Redsn0w then open iTunes 10.5 downloaded above and you should get your iPhone in Recovery Mode message.

STEP 7: Now hold the Option key on Mac and hit Restore. Point iTunes to your iPhone 4 Custom iOS 5 IPSW which begins with NO_BB. (WARNING: Don’t select the stock one or you’ll lose your unlock)

Finally, you will need to use Gevey SIM with the way you knew. In case you’re not aware, here’s a video showing how to use Gevey SIM on iPhone 4: