There has been some discussion of Diablo 4 being announced this year due to what some are saying is just a conspiracy theory and not creative advertising on Blizzard's part. Rhykker has posted up a video going over the item that is causing the uproar. I recommend checking it out.
Are we seeing and believing something solely based on our extreme want of seeing something new come to the Diablo world? Blizzard has shown before that they can pour out the creative juices for advertising although it isn't something they do often.
It shouldn't be surprising to anyone by now that something is going on at Blizzard regarding the Diablo universe. We have seen a lot of people either leave or swap teams recently as well as job postings for a new project in the Diablo universe. Trying to get a Blizzard team member to discuss the future of a game can be very challenging. Most of the time, the individual will either say I don't know or that they cannot talk about that subject. Usually when Deani and I would broach the subject, it would make the individual nervous, so we never pressed the issue. The only Blizzard employee that felt comfortable discussing future "plans" of Diablo was Wyatt Cheng and he was always very careful with his information delivery. A lot of the time it started with "Here is what I can say" and a long pause as he ran through the information in his head making sure it met all the checks.
I for one hope the conspiracy turns out to be a piece of mad genius advertising and if Diablo 4 really is going to make it's appearance at Blizzcon, I need to find out where I can get a press pass!