Section23 Films' Chris Oarr commented that only the first two seasons were licensed, with an option on the rest
source wiki.
why can't people do their own research? It took me 1minute to to find this line with google/wikipedia.
I'm only guessing here, but it's safe to assume that the dvd sales weren't high and so maybe sentai filmworks decided against picking up further seasons. you know, common sense.
I might be wrong and they will pick it up later. on their site i see 48 released episodes, which falls short 4 episodes of the two licensed seasons, assuming we're talking 26-episode seasons here.
to further answer your question about the release shedule. *sigh* looking at amazon.com you can easily deduct that the release shedule is more than jumpy at best. first dvd was released APR 2010, second dvd on JUN 2010 and then a jump to third dvd on DEC 2010. The forth dvd got out in Feburary this year. It's up in the air and anyone's guess.