In an interview with The Telegraph at BlizzCon 2019, Diablo 4’s executive producer Allen Adham was quoted as saying that cross-play multiplayer support is something that the team is working towards. What this means is that PS4, Xbox One, and PC gamers will be able to play together all at once.
According to Adham, “We’re very excited about cross-play. There are technical details and details to work through with the first parties, but it’s our goal to get to cross-play.” This will be particularly important for Diablo 4 due to the nature of the game, which some have described as almost like an MMORPG.
With Diablo 4, players will be able to encounter other players in the world as they travel, even if you aren’t necessarily in a party. There will be world events which means that players can band together to fight bosses or participate in those events, so having a larger pool of players to play with will no doubt benefit the game.