The only way to downgrade is to restore a backup (Which you should always make before upgrading) or re download one of the cracks that was th version your looking for.
restoration of baack-up is only way unless you request for it http://www.mpgh.net/forum/421-minecr...craft-jar.html a recent 1.4_1 jar i gave keep reading down to see the attachment
Yeah just use the thread that Jailbroken posted, I used it and now I can re-connect to my server, (and i banned a griefer at the same time )