It seems like Blizzard has started the development of the next game in the Diablo series, as they are now looking for a
Senior Concept Artist for an "Unannounced Project" within the Diablo universe on their home page.
They specify that the person they seek must be "exceptionally skilled" and should "drive the vision of characters and worlds in our next hit game!"
The person should also be one who "loves dark medieval worlds, and feels there are countless stories left to tell in the Diablo universe!"
Creating concept art for a game is often one of the first stages of development, so it might just be that the studio is starting to think about a sequel now. Of course, things might be much further along than that.
Of course, we don't really know if this is Diablo IV at all, it might be something completely different. Even if it is, knowing Blizzard it could take many years before we actually get any information. Then again, Diablo III was released in May 2012, which is an eternity ago in this business.
Back then we didn't know anything about PlayStation 4 and Xbox One, Borderlands 2 wasn't released and we still hoped Resident Evil 6 would save the horror genre - and Star Wars was strangely enough owned by a bearded man known as George Lucas. Innocent times, indeed.