There was no Book 4, so no DVD can be released. The Last Airbender TV series ended with Book 3, but the story continued in a comic-book series 'Avatar: The Last Airbender: The Promise'. A spin-off TV series set in the same universe but many decades after the events of The Last Airbender called 'The Legend of Korra' is ongoing.
It's already out
Yes, you can buy the DVD for Avatar Book 2 in Australia.
The complete "Avatar: The Last Airbender: Book Three" came out on DVD September 16, 2008.
the DVD is supposed to come out before it aires, and the DVD is set to be released July 29, but the episodes might get leaked, keep checking avatarchapters.org
it came out yesterday
It's already out on DVD and Blu-Ray
Avatar will be releeased on DVD and Blu-Ray on April 22, 2010
DVD release AUGUST 16 2010, only at wal-mart.
Thursday, April 22, 2010
It is Coming out on 26th April 2010
Hopefully never. Read the book instead.
The entire series is out and available to buy on DVD now. If you are still interested to know the first part of Book 2 was released January 27, 2009.