There was an article in some english news some days ago where one of the developers said there "might" be plans for WC4.
Can't find it at the moment. Sometimes i read to much :-).
I mean they teaser that thing since years you're right with that fact but i don't think its that far away.
With the new brands of Blizzard like Hearthstone, Overwatch, Heroes, they are showing that they focus more on e-sports and
that kind of stuff.
A WC4 would fit in so perfect.