The fifth Stormlight book is coming out in December.

The way Sanderson has structured it, is that Stormlight will have ten books, but that the fifth book will be a major climax with a long time gap until book six which will start a later story. So it's a ten-book series told in two parts, and the climax within part five is coming up soon.

I've read through book four, like 4800 cumulative pages, and am locked. That's the first fiction book I'll buy on day one.