From what I know, there WILL be a fifth series, and it is supposedly about the early clans. I am not sure whether it is about the start of the clans, the ancients or something else entirely, I just know that it will be before the gatos came to the lake and before Firestar was born. In my opinion, I want warrior gatos to keep going on, even though the warrior cat ideas are getting crazier (Supercats!?).
And even though I will miss the characters we have grown to know and love, I'm pretty excited that we get to see either the ancients that used to live at the lake, the start of the clans or something else with lots of new characters, new personalities and a awesome plot.