Chronic Dev Team’s greenpois0n RC5 was released a while ago. Here is greenpois0n 4.2.1 tutorial that will help you jailbreak and untether iOS 4.2.1 on iPhone 4, 3GS, iPad and iPod Touch.
How to Jailbreak iPhone 4 4.2.1 with Greenpois0n
Before you Begin:
- Backup your data
- Install a fresh copy of iOS 4.2.1 to your device (Recommended)
- Download [Greenpois0n RC 6.1] Greenpois0n RC5
Step 1: Connect your device to the computer via USB and launch Greenpois0n RC5
Step 2: Click on “Jailbreak” button in Greenpois0n
Step 3: Follow on-screen instructions to put iPhone in DFU mode, you’ll need to:
- Press and hold the Power button for 3 seconds
- While holding the Power button; also press and hold home for 10 seconds.
- Release the Power button; continue holding the Home button for 15 seconds.
Step 4: Greenpois0n RC5 will now process the iOS 4.2.1 jailbreak.
To Install Cydia:
After jailbreak, you’ll find Loader on iPhone Springboard. Tap on it to install Cydia.
That’s it. Enjoy the untethered iOS 4.2.1 jailbreak with animated custom boot logo.
kunal says: 
Yoe81 
