When will the iPod Touch 4th generation be jail-broken?
Currently, there is only a tethered jailbreak that works well. Dev team has a beta redsnow untetherd jailbreak that only works on macs and is a little buggy. There should be an untethered jailbreak by the en of this year.
does black rain not work?
Yes, on 3rd gen you have to jailbreak it to have a background on your homepage, on 4th you do not.