The following guide will help you to know that how to jailbreak iPod Touch 4G, 3G untethered running over iOS 4.2.1 with Greenpois0n RC5.
Warning: Users who rely on Ultrasn0w unlock must update to stock 4.2.1, otherwise stay away from Greenpois0n RC5 tool as they will lose the unlock.
Note: This guide is written for Mac OS X users.
Jailbreak iPod Touch 4G/3G Untethered on iOS 4.2.1 with Greenpois0n RC5
Create a folder on your desktop, download Greenpois0n for Mac, and unzip it in your created folder, suppose with the name Untethered Jailbreak
Double click on Greenpois0n icon to launch it inside your folder
Attach your iPod Touch to Mac, power it OFF, click on Jailbreak button on Greenpois0n screen
Greenpois0n will instruct to put your iPod Touch into DFU Mode, so continue with those instructions carefully
When your iPod Touch will enter into DFU mode successfully the jailbreak process will automatically start and on completion of process just click Complete! button
After restart of your iPod Touch you will find Greenpois0n Loader app sitting on Springboard, so tap on it to launch that
Select Cydia there, and tap on Install Cydia button
Once Cydia will be installed, it will prompt to remove Loader from your iPod Touch, so tap on Remove button to do that
Reboot your iPod Touch by pressing Home button
Enjoy, your iPod Touch 4G/3G over iOS 4.2.1 is successfully jailbroken with Greenpois0n RC5.