Well, there isn't an Aurora Rising 4 per se'. However there will be 6 more books.
The next 3 books are called "The Aurora Renegades" series. The author is only 50% through the first draft ATM.
Book titles will be Sidespace, Dissonance & Abysm. The final 3 book series is called "Aurora Resonant". She's capable of producing a book every 6 months so you should at lease see the eBook version of Sidespace in December/January. (?)
Books 0.5 (Restless Vol. I) was never made into an audible book.
However you can get both .5 & 3.5 (Restless Vol. II) for $7.00
but not in audiobook format.
LOL How's that for an emotional roller-coaster ride?
You actually get all 5 books for that price here...
http://www.audible.com/search/ref=a_search_c4_1_1_1_srAuth?searchAuthor=G.+S.+Jennsen&qid=1438037129&sr=1-1Dallis♥