The following guide will help you to know that how to jailbreak iPhone 4, 3GS untethered on iOS 4.2.1 with Greenpois0n RC5, which is recently released.
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 iPhone 4/3GS Untethered on iOS 4.2.1 with Greenpois0n RC5
Create a folder on your desktop, download Greenpois0n RC5 for iPod Touch, 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 iPhone into DFU Mode, so continue with those instructions carefully
When your iPhone will enter into DFU mode successfully the jailbreak will get start and on completion of process just click Complete! button
After restart of your iPhone you will find Greenpois0n Loader app sitting on Springboard, so tap on it to launch that
Select Cydia and tap on Install Cydia button
Once Cydia will be installed, it will prompt to remove Loader from your iPhone, so tap on Remove button to do that
Reboot your iPhone by pressing Home button
Enjoy, you have jailbroken iPhone 4/3GS over iOS 4.2.1 with Greenpois0n RC5.