I actually read somewhere long time ago, with the release of Lich King, that the Warcraft series is not complete, there is so much more to fill in.
A Warcraft 4 would obviously fill those gaps, making the series complete (With the final island, Dwarf Mines or whatever it was named, and the death of Sargeras.)
It has to come sooner or later, else they won't be able to make another expansion to WoW. It would be dumb to add stuff that has only been mentioned in Warcraft books, and not in the Warcraft games.
If I remember correctly :)








