Dev Team has also released GreenPois0n RC5_2 for Mac. It contains fixes of various issues occurring in GreenPois0n RC5. GreenPois0n RC5_2 can also be used as untethered jailbreak for iPhone 4, iPhone 3GS, iPod touch 4G, iPod touch3G and iPad running on iOS 4.2.1 without SHSH blobs of iOS4.2b3.
Below are the instructions on how to Jailbreak iOS 4.2.1 untethered on iPhone 4, 3GS, iPad, iPod Touch 2G, 3G, 4G and Apple TV using GreenPois0n RC5-2.
Step 1: Download the Mac OS X version of GreenPois0n by navigating to: http://www.greenpois0n.com.
Step 2: Now save your required download to any location.
Step 3: Extract the GreenPois0n tool archive on desktop and double click on greenpois0n to launch.
Step 4: Connect your iDevice to the computer and then click on the Jailbreak button.
Step 5: On this screen you will be asked to place your phone in DFU mode. You can either found some help in our DFU tutorial or follow the instructions on the given screenshot.
As instructed in the screenshot press and hold the sleep button for (3 seconds)
Step 6: Continue holding sleep; press and hold home for (10 seconds)
Step 7: Now release sleep button; continue holding home button for (15 seconds)
Step 8: Once your your iPhone is in DFU mode. Jailbreaking will start.
Step 9: Now finally press the complete button as you have successfully jailbroken your device
Step 10: Once your iPhone will reboot you will notice a new Loader icon on the Springboard. Tap it to launch.
Step 11: In loader, tap Cydia and then again Tap large Cydia button.
Step 12: Once Cydia is installed successfully it will prompt you if you want to keep the Loader or not. Just tap remove button as you don’t need it for anymore reason and then tap home button to restart your iPhone.
Step 13: After your iPhone will restart successfully you will see a Cydia icon. Tap it and do all the essential updates required. Congratulations you have successfully jailbroken your iPhone untetheredly on iOS 4.2.1.