Step-by-step instructions of jailbreaking iOS 4.2.1 with Greenpois0n RC5
1. Download Greenpois0n RC5 for Mac OS X.
2. Connect your iPod Touch or iPhone to iOS 4.2.1 firmware to USB.
3. Start Greenpois0n RC5 App.
4. Click on Jailbreak Button and follow the on-screen instructions so that you can put your iPod Touch or iPhone in DFU mode.
5. When you have your iPhone or iPod Touch with iOS 4.2.1 in DFU mode, click on “Jailbreak” on Greenpois0n to start the jailbreak process.
6. When Greenpois0n finishes jailbreak process, a new icon “Loader” will appear on Springboard. Click on Loader to install Cydia
7. If you don’t see Cydia, reboot your iPhone/iPod Touch.
You have successfully jailbroken iOS 4.2.1 untethered on iPhone 3GS, iPhone 4 and iPod Touch 2G / 3G / 4G.
There’s no unlock available for iOS 4.2.1 and if you want to unlock it, don’t update to iOS 4.2.1.