Last week, Twitch streamer and tipster Metro claimed that Blizzard would release Diablo 4 alongside Diablo 2 Remastered at BlizzCon 2019. Now, this has seemingly been indirectly confirmed by Blizzard in an advert for a new Diablo artbook.
https://www.notebookcheck.net/Diablo-4-leaked-New-artbook-with-photos-from-Diablo-1-2-3-and-4-released.439948.0.html