Members of Chronic Dev Team has finally announced the release of the long awaited GreenPois0n RC5. This tool brings untethered iOS 4.2.1 jailbreak for all iOS devices. Besides you do not need to have ‘4.2b3’ SHSH blobs saved in Cydia, unlike the Redsn0w 0.9.7. Keep in mind, the software is only available for Mac OS X. A Windows version will be coming very soon.
Follow the below guide for step by step instructions to convert your tethered jailbreak to untethered jailbreak on iOS 4.2.1:
This is it! You are done ! Download GreenPois0n Download iOS 4.2.1 for iPhone 4, 3GS, iPod touch 4G / 3G / 2G and iPad
STEP 1: Your device must be on iOS 4.2.1, then download GreenPois0n RC5 for Mac OS X from here.
STEP 2: Launch GreenPois0n RC5 then connect your iPhone into your Mac via USB and click on Jailbreak as shown below:
STEP 3: Follow the instructions shown on the screen to enter DFU mode:
STEP 4: After the jailbreak is done, you will have a new application on your springboard called Loader. Use it to install Cydia, then it will ask you if you want to remove it. A quick re-spring, and you now have Cydia.