The Chronic Dev Team has just released the greenpois0n new version which will allow you to jailbreak the iOS 4.2.1 once and you can finally reboot the device without having to re-jailbreak.
By using greenpois0n you will be able to free your device from its limitations. But as a word of precaution, as it has just been released today, so there may be issues. So we should be careful enough and report the bugs.
Greenpois0n is now available for both Windows and Mac on their website. Due to heavy traffic, the site may not be available sometimes. If you want to download, you can always search for alternative mirrors in Google.