On Blizzard's careers page, the developer lists almost two dozen positions for an unannounced Diablo project. Among these, is the role of quest designer. The job listing states that the Diablo development team is seeking a content designer that is capable of helping the team "create play areas, quests, characters, and dialogue for an upcoming project." Candidates should also have a strong understanding of quest design and key gameplay concepts, as well as the lore of the Diablo franchise and Blizzard game franchises in general.
Other positions listed by Blizzard within the Diablo franchise include environmental artist, senior lighting artist, senior software engineer, senior systems designer, and technical game producer. There are 19 jobs listed in relation to this unannounced Diablo project. Jobs related to the Diablo Immortal mobile game are labeled separately.
The job listings do not explicitly state that Blizzard is working on Diablo 4. It's also important to note that following Blizzcon 2018, Blizzard reaffirmed that it has "multiple Diablo projects" in the pipeline as well as "multiple" teams working on the franchise. But, it's not exactly outlandish to suggest that one of these games is Diablo 4, considering how successful Diablo 3 was and how eager fans are for a follow-up.
It's also important to consider that following the announcement of Diablo Immortal, Blizzard faced waves of backlash. Fans were that upset by the reveal of the mobile game that the announcement trailer for the game was mass disliked on YouTube. Many criticized the company for the reveal, saying it didn't know what fans really wanted, and its stock price plummeted too. A reveal of Diablo 4 would give fans what they want and would help to get them back onside after such a disastrous Blizzcon.
However, it's unclear exactly when Diablo 4 may be announced. If all 19 of these Diablo job listings are related to Diablo 4, then this would indicate that the game is still very early in development. This means that it could be a long way off from release or even an official unveiling. Some progress is better than no progress, though, and fans will have to be patient until Blizzard has something official to share.
Diablo 4 is rumored to be in development for unspecified platforms.