I don't believe Blizzard will ever announce "Diablo 4". I could be totally off, but my feeling is that with Diablo 3 and Starcraft 2, they have come to realize that making sequels in this way limits what they can experiment with and what fans have come to expect from sequels to "classic" Blizzard games. While they have certainly turned Diablo 3 around, announcing Diablo 4 now will make fans wonder if it is too early and if they're starting to "milk" the franchise with a quick sequel. Another Diablo 3 expansion would just be diminishing returns at this point.
On the other hand, looking at Starcraft 2's Nova Ops, Hearthstone, Heroes of the Storm, and Overwatch, I think Blizzard is transiting completely into a more episodic seasonal service-driven product line. It makes sense to me then, that alongside remasters of older Blizzard games, they would also want to reboot Diablo as a product to something which they can get out quicker, get more feedback from fans, and push through new content at a regular basis. So I don't think it'll be "Diablo 4" so much as a brand new Diablo.