While it’s not “official”, Blizzard is looking for a new Diablo director. According to the site,

Blizzard is seeking a game director with outstanding communications skills, proven experience in creative direction, strong ability in system design, and a stellar track record of shipping AAA games to lead the Diablo series into the future.

Diablo III has been out for quite some time, especially with the PC version since it’s 2012 release, while the Xbox 360 and PS3 versions were out in 2013, and for the Xbox One and PS4 in 2014. Game development does take a few years, so it is most likely for a 2019-2020 release. Project Scorpio, the PS Neo, Nintendo NX, and the PC are the strongest candidates for the game’s respective platforms.
Again, the announcement of the game is not “official”, but the official job listing does contain the Diablo name, so it’s a safe bet there will be a Diablo IV.