[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]