yeah it says in the back of the third book that he is writing the fourth book as he couldn't fit it all in 3..

Originally The Inheritance Cycle was going to be a trilogy, but, while writing the beginning of Brisingr, he realized there was going to have to be an extra hundred pages, which made Empire (what it was going to be called) too big, so he split it in half and created the cycle, Brisingr, and the fourth book, instead of a trilogy and Empire.

Yes it said in the back of the 3 book