Redsn0w beta will JB 4.3 if you point it at the 4.2.1 ipsw. But you'll still need a different tool to boot it into the jailbroken kernel. Or you can grab a bundle, and use Pwnage Tool to make a custom ipsw and preserve your baseband.
Either way, its a tethered jailbreak. But the jailbreak monte will still be able to be used once they update redsn0w.