After the Star Wars Rebels panel at Star Wars Celebration Orlando, the trailer for Star Wars Rebels season 4 was posted. Sadly, the 4th season will be the show's final season. Apparently the season will be quite a short one at 15 episodes. The first episode will be an hour-long event (likely a two-parter, and if the pattern holds, that might happen for the finale as well, making the season even shorter).
The tagline for the season 4 trailer is "Destinies are revealed in the final season of Star Wars Rebels."
Star Wars Rebels season 4 starts this Fall on Disney XD.