This was also discussed on Warseer and they say 3 month for
BL hardback novels to be released in softback. So ET4 focusing on Skaven in January (confirmed by Arthurius11) and ET5 in March
I will look for the topic, 2 minutes
*Edit* Found the thread
http://www.warseer.com/forums/showthread.php?402823-End-Times-Books-4-and-5-Timing-and-BL-authors
And, yes, it will be hardcover novel + supplement at the same time