Sn0wbreeze 1.6.1 are similar to Pwnagetool, by using Sn0wbreeze you can activate custom recovery logo, multitasking, unlock etc. Now we can Jailbreak Iphone 3g and iphone 3gs on iOS 4.0 with complete step by step guide to jailbreak iPhone 3G & iPhone 3GS 4.0 iOS using Sn0wbreeze 1.6.1 on Windows .
How to Jailbreak iOS 4.0 with Sn0wbreeze 1.6.1 on Windows

1. Download iPhone 3gs/3g 4.0 Firmware , Download iTunes 9.2 , Download Sn0wbreeze 1.6.1 for Windows

2. Make a new folder named  “ios4.0”  and put everything in that folder.  Open sn0wbreeze 1.6.1 > “blue arrow” > select “simple mode” and continue.

3. Browse and select iOS 4.0 ipsw file downloaded in step 1 for your respective device. Sn0wbreeze 1.6.1 will process 4.0 ipsw and will verify select ipsw file.

4. Click on Blue arrow icon and wait for Sn0wbreeze 1.6.1 to complete processing, Now Click yes/ no if you want to activate iPhone.

5. Let Sn0wbreeze 1.6.1 to complete rest of its process, after completing the process select whether your iPhone 3G is jailbroken  or not.

6. Now connect your iPhone 3G and enter in Recovery mode after that launch itunes and hold down shit while clicking the restore button on iTunes.

7.let iTunes complete installing iOS 4.0 custom firmware on your iPhone 3G/3GS. Once your iPhone restarts, the jailbreak process will complete.