Jailbreak iOS 4.1 on iPhone 4 or iPhone 3GS
Create a folder on your desktop, for example with the name Greenpois0n Jailbreak. Download Greenpois0n for Mac and keep it in your created folder
Open the folder Greenpois0n Jailbreak and unzip gp_mac_rc3.zip downloaded file. Then click on extracted greenpois0n icon to launch the application
Connect your iPhone 4 to your Mac, Power OFF iPhone and click the large Jailbreak button to continue
It will display instructions to put the iPhone to DFU Mode, so follow those carefully.
First, Press and hold the sleep button for 3 seconds
Then, Continue holding sleep; press and hold home button for 10 seconds
Release sleep button; continue holding home button for 15 seconds
Now, wait a little as it will process to put the iPhone to DFU mode and when it will done the jailbreak process will get start
On completion click Quit button
Now, your iPhone 4 will restart. You will find Loader icon on your iPhone’s Springboard, tap it to launch
Tap to Cydia > Install Cydia button and let it complete
Now, it will prompt to remove Loader so tap on Remove button and then press Home button
Now, your iPhone will reboot so tap on Cydia to let it grab the updates
Enjoy! Your iPhone 4 on iOS 4.1 is jailbroken with Greenpois0n. Same guide works to jailbreak iPhone 3GS on iOS 4.1 for Windows.