Now guys, to run Cydia on Unlock iPhone 3G 4.0 OS beta 5.13.3 baseband, fuzzyband can be use for downgrading and unlock iPhone 3g 4.0 OS beta 5.13.3, Yes iPhone 4.0 OS beta baseband 5.13.3 downgrade with fuzzyband have to be done to run Ultrasnow 0.92 via Cydia, eitherwise Ultrasnow will not run on the iPhone 4.0 OS.
Few things which need to be mentioned here are the conditions. The conditions are this downgrade will only work if you have iPhone 3G, bootloader 05.08 with 5.13.3 Baseband. So guys if you want to try it on other baseband or bootloader, you still try it but only at your own risk.
Procedure is again very easy to Unlock iPhone 3G 4.0 OS Baseband 5.13.3 :
* First you must Jailbreak your iPhone 4.0 OS beta with RedsnOw 0.9.5.
* After you Jailbreak, find Cydia from your springboard and acfter launching it, search for the “Fuzzyband” App and install it.
* Launch fuzzyband after installation and  hit the “Downgrade” button. And downgrade will be done from 5.13.3 to 4.26.8 baseband.
please note that to run UltrasnOw 0.92, you must downgrade your iPhone 4.0 OS beta with fuzzyband and then only you can unlock your iPhone with UltrasnOw. The procedure of Jailbreak / Unlock with UltrasnOw 0.92 is same for the iPhone 4.0 OS to that of iPhone 3.1.3 OS
For other Bootloaders, simply launch Fuzzyband and it will tell you the bootloader of your iPhone and if it is compatible, it will downgrade it as well.
The instructions will only help unlock iphone 3g by downgrading iPhone 3G which is currently on iPhone
OS 3.1.3 with 05.12.01 baseband and older bootloader 05.08. This wont work with iPhone 3GS.
If your iPhone meets the above criteria, you can then use the latest version of Fuzzyband which supports iPhone 3.1.3 to downgrade your 05.12.01 baseband to the older 04.26.08 even if you accidently upgraded to the stock iPhone OS 3.1.3 from iTunes. Once you have downgraded your baseband, you can then use ultrasn0w to unlock your iPhone on iPhone 3.1.3 firmware. Here’s how you do it.