But there is the unwanted news along with the wonderful one, because Cydia store will not be used albeit your iOS 4.2.1 is jailbroken, if you run Cydia - it will crash. This is happening because Apple deliberately has altered the private APIs that Cydia had been used.
Thus, even thought you managed to jailbreak the iOS 4.2.1 for your iDevices it won't benefit you much, each time you wish to download and install the jailbreak apps from Cydia, you need to install is manually with SSH.
But if you insist to jailbreak anyway, these are the requirements you need to have:
- Download iTunes 10.1 for Mac and Windows
- Download Redsn0w 0.9.6 Beta 2.
- Download iOS 4.2.1 GM
- Download iOS 4.1 (Final version) for iPhone 4, 3GS, 3G, iPod touch