A new series is in the works for Hajime Kanzaka's Slayers fantasy light novels. The series will span 6 months, (26 episodes I'm guessing) but there is no word on when we will see it. It will be directed by Takashi Watanabe who will oversee J.C. Staff, who produced the three previous seasons. There was no confirmation on whether the original cast will once again be featured or if Lina will be the lead character once again.
Funimation and Central Park Media both released the three previous seasons in English in North America.