Update: Blizzard has issued a statement denying that it had intended to announce Diablo 4 at BlizzCon. "We generally don’t comment on rumors or speculation, but we can say that we didn’t pull any announcements from BlizzCon this year or have plans for other announcements," a rep said in an email. "We do continue to have different teams working on multiple unannounced Diablo projects, and we look forward to announcing when the time is right."
Kotaku has updated its report to confirm that the video of Blizzard co-founder Allen Adham discussing a new, full-scale Diablo project was made; what isn't confirmed at this point is whether it was actually intended for BlizzCon, or if it was simply an idea that was kicked around and then discarded. Whatever the case, the site said that Diablo 4 is in development, but it sounds like it hasn't gone perfectly smoothly so far: It has "heard" that the new Diablo has gone through two iterations under different directors over the past four years, and may change further as development continues.
The announcement of Diablo Immortal at BlizzCon went about as badly as it could have. The reveal of a mobile game rather than a full-on Diablo 4 led to a "huge backlash" at what is traditionally a two-day celebration of all things Blizzard. According to a Kotaku report, however, the reaction to Diablo Immortal's announcement could've been very different, as the site says that Blizzard planned to confirm Diablo 4 is in development but bailed on the reveal before last Friday.
Two sources familiar with the situation told the site that the original plan had been to announce Diablo Immortal, and then end with a video in which co-founder Allen Adham would confirm that Diablo 4 is in the works, but not yet ready to show. The move sounds like it would have been very similar to Bethesda's Elder Scrolls 6 announcement at E3, an entirely insubstantial statement of the obvious that nonetheless had fans in a much happier frenzy.
But for reasons unknown, the plan changed a couple of weeks prior to BlizzCon, and the Diablo 4 reveal was scrapped. One of Kotaku's sources said that Blizzard decided it wasn't ready to commit to the announcement because development is still in a state of flux, and the game may continue to change as it progresses.
That in itself doesn't really explain why Blizzard would decide to pull the plug on the reveal, though. It's not as though Diablo needs to be explained at this point—like The Elder Scrolls games, each new iteration is changed but the core experience remains essentially the same. That's how Bethesda was able to draw roaring applause by simply splashing four words onto a big screen at E3, and Blizzard could easily have done the same thing: Tell the audience, "One more thing," smash a Diablo 4 logo on the screen, "Thank you! Goodnight!"
I've reached out to Blizzard for more information and will update if I receive a reply.