[HOW-TO]: Download and Jailbreak iOS 4.1 Custom Firmwares iPhone 4 / 3GS / 3G

1. You can take this risk to download the customized .ipsw files
2. But it will certainly helps you in the Jailbreaking process

[Source: Covering Web]

STEP 1: Download the iOS 4.1 custom firmware for your device at the end of the page. Also download and install iTunes 10. STEP 2: Join the splited custom firmwares you downloaded using BetterZip for Mac OS X or HJSplit for Windows to make them only one .IPSW file. STEP 3: Start iTunes and put your iPhone into DFU mode by following this steps:

• Hold Power and Home buttons for about 10 seconds
• Then release the Power button but without releasing the Home button for 10 more seconds
• Your device should now be into DFU mode

STEP 4: Now, You will have to restore your iPhone to the jailbroken custom firmware by Clicking on the iPhone icon on the left sidebar of iTunes then hold left �alt� button (�Shift� button on Windows) on the keyboard and then click on �Restore� then release the alt button, now Select the custom firmware you already download and let iTunes go in a bunch of process.

STEP 5: After your iPhone is restored and jailbroken successfully. Follow our guide posted here to unlock iPhone 4, iPhone 3GS and iPhone 3G using Ultrasn0w carrier unlock. 

iPhone 4 Custom Firmware, Activated [File 1, File 2, File 3]
iPhone 3GS Custom Firmware, Activated [File 1, File 2]
iPhone 3G Custom Firmware, Activated [File 1, File 2]