I'm willing to bet money that we will not get a Diablo 4, for a LONG time. With, that is, one very specific exception in my opinion.
this picture greatly disappoints me where now i'm thinking d3 expac where they need to cut their losses and make a new game, theres too much wrong in d3 to make it worthwhile, engine is too old, graphics are meh (lost ark says hi) greater rifts are a mess, itemization is shit. skills are old and boring. the list goes on and on
d3 is dead. and it should stay dead.
You see, keeping Diablo the way we know it ( single player or only a couple people, pretty much everyone playing their own instance of the game ) would mean there's zero reason to scrap an ENTIRE game. This argument of 'the engine is old' is ridiculous. Blizzards not going for ultra uncanny valley realism, so there's no reason to create and entirely new game with a new engine to boot. Every problem can be fixed via patches, and all an expansion is is just a huge patch. Blizzard is not going to just discard ALL the artwork, world building ect just to fix some problems.
The only possible way Blizzard would use up so much time and resources to make a brand new Diablo game on a new engine, would be if it plays drastically different than what Diablo 3 is now. If, for example, they wanted to make a Dark Souls type game, then they would need all new art, systems ect. OR, if, (he says in hushed tones) they wanted to try a Diablo style MMO, then again we'd see everything built from the ground up.
So if Blizzard wants to stay in the isometric / single player or small group / instanced game / demon slaying loot hunt genre that Diablo is known for, at MOST we'll get an expansion announcement.
But, if they want to go a brand new direction for Diablo, THAT'S when they'll make an entirely new game.