Blizzard Entertainment's career page reveals nine active listings for new employees on an "unannounced project" within the Diablo franchise. They aren't necessarily bound for Diablo 4, but since the game is unannounced, the positions must not be for work on the mobile Diablo Immortal game that was revealed at Blizzcon 2018.
This job listing includes openings for an executive producer, lead character artist, and lead concept artist, all of which are fundamental roles in the creation of a modern, big-budget video game. Those positions being unfilled (or seeking new talent) implies that the unannounced Diablo project is still a number of years away from completion.
According to a fan translated report Diablo 4 has already been shown off in some form to teams at Blizzard Entertainment, along with internal confirmation that a new Overwatch game is also in development. That report lines up with an earlier rumor that Diablo 4 and an Overwatch sequel were still on the way after Blizzard canceled the secret work on a StarCraft Online Shooter.
If it feels like the wait for Diablo 4 is interminable, just remember that there was a nearly 12-year gap between Diablo 2 and Diablo 3, and also that WarCraft RTS fans haven't gotten a new one of those since 2002. Blizzard taking its time is one of the sweet sorrows of enjoying the studio's franchises.
https://www.gamesradar.com/diablo-4-job-listings/