How to Jailbreak an iPhone 4 I.O.S 4.2.1 With Greenpoison

Prerequisite :  -Have this iDevice : Iphone 4 with firmware 4.2.1.
Download Greenpoison here

 

 STEPS

1. Unzip the archive gp_win_rc5_b2.zip
2. Connect the iPhone and switch off it.
3. Click Prepare to jailbreak DFU.
4. Follow the procedure:-Press 2 seconds on the button Power. 
5. Then without releasing(leaving) the button to power,  maintain the button home during 10 seconds.-Then relaunch the button Power and remain  press on the button home during 15 seconds.
6. If the Ready to jailbreak message appears, then  click to Jailbreak
7. The jailbreak start, the screen becomes quite white 2 seconds then writings on a black background appear.
8. Click Quit at the end.
9. When the menu of the iPhone reappears, look for the Loader application and  install Cydia

TIPS

If the Try Again message appears then you are not correctly spent in mode(fashion) DFU. Put out(switch off) the iPhone and restart.

WARNINGS

As usual, there is a risk of having to restore if Jailbreak happens badly. It is  very important to make a complete saving of the iDevice before beginning the manipulation.

Source : WikiHow