how to jailbreak itouch 4.2.1?

It depends where you got your itouch 4.2.1, if you got it through an authorized dealer,they will be able to fix that problem for you easily. If you got it second handed or from any third party companies, then you have a problem. The most common problem with itouch are the "unlock" request, Apple do so to keep their phones lock for a reason, but there are lots of programs out there or even business services that will help you unlock/active your itouch. Check Google for itouch unlock or activate itouch, trust me its super easy. And if it cannot be solved check these if it’s still jail broke go to cydia and install ultrasn0w,if its not jailbreak with redsn0w and then cydia.,if you have the newboot ROMs you need to install the itouch baseband to unlock. Or else you cancheck websites which help the phone to be unlocked easily.