While Jailbreaking your iPhone, iPod, iPad with the GreenPois0n Jailbreak 4.2.1 you will get a loader app on the springboard of your iPhone, iPod, iPad. Loader App helps you to install Cydia on your iPhone, iPod, iPad, but after installing Cydia, Loader App become useless.
Once you have successfully Jailbroken your iDevice 4.2.1 with the help of Greenpois0n tool. You have to follow a simple tip in order to install cydia in your iDevice. Follow the tutorial below to install Cydia after GreenPois0n Jailbreak 4.2.1.