During BlizzCon 2019 Angela Dell Priore while answering a question confirmed Diablo 4 is an online-only game across different platform. Diablo 3 also required a internet connection while playing but the PS4 edition offered a offline gameplay which is not a part of the next Diablo 4 serires.
Following is an reply by Angela Dell Priore – Lead Designer:
We are not going to support an offline mode,” Del Priore replied. “But as I said before, nothing in DIablo 4 is going to require partying up. You can play solo and dungeons are private. Campain quest areas will [also] remain private.
Diablo IV similar to its predecessor is an fully open world game, players can explore five distinct region (Scosglen, Fractured Peaks, Dry Steppes, Hawezar and Kejhistan) in any order of their choice. Player’s can choose to go solo or party up with their online friends. Few more details came out from yesterdays’s interview highlighting some core aspect of the game. Like Dungeons will remain private for Solo or partied players while online in Open World you will encounter public. Diablo 4 allows you to set the difficulty level prior entering into the dungeon.
Diablo 4 does not comes with any option to to disable other players or switch to offline mode. But when you play Solo you can have some privately progress through various levels.
Announced at BlizzCon 2019, Diablo 4 will be coming for PC, PS4 and Xbox One. Release date is not confirmed.