Luke Barnes will be returning for Season 4 as his role of Rast. Rast, a member of the Nights Watch, was last seen in Season 3's episode 4, "And Now His Watch Is Ended," during the mutiny at Craster's Keep, in which he killed Lord Commander Jeor Mormont.
Rast's role in the show departs from his role in the books. In the books Rast was given orders from Bowen Marsh to defend Castle Black. During the Battle of Castle Black Rast was killed by a Thenn.
Since non of that can happen, as I doubt Rast will be returning to the Wall anytime soon after killing the Lord Commander. What his role will be in the book, besides a new point of view north of the Wall, is anyone's guess.
Altogether I am very excited to see how this plays out. It will be nice to get a new perspective, that was never included in the books.