Well according to this guy the GreenPois0n jailbreak for this operating system version will be tethered or semi-tethered.
As a matter of fact even the dev team had confirmed that iOS 4.2.1 jailbreak and unlock will be around the corner pretty soon.
The earlier devices like the iPhone 3G and 3GS were most likely un-tethered and unlockable but however technology gets more advanced due to these guys who work hard to make it more sophisticated.