Season 2 part 1 comes out in february with part 2 later in the year.
Around March/April Season 5 aka the lost season will be released.
Season 6 (fast forward) is available in two parts as well.
There is a "season 4" set but it ust includes random episodes from season 4, i'd wait until a real "season 4 pt. 1 and 2" comes out. and don't bother with any of the single volumes for the new toon.
This show is highly recommended, the original show is also available on dvd with volume 1 (season 1), volume 2(season 2), volume 3-6 (season 3), Season 4 set, and season 5 set.