Some new details were shared regarding how exactly the open shared world of Diablo 4 will work.
There will be no game-wide difficulty levels because of the shared nature of the open world. Enemies will scale so that you and your friends can play together. Keep in mind, some areas will still be more difficult than others, so plan accordingly.
Dungeons can be private for solo or party play. The open world is the only place where you will come across other players. Additionally, player population will be automatically ramped up or down,
“To maintain the sense that you're an embattled hero in a desolated environment, player populations will change depending on what part of the world you're in. Towns will be the most populous. As you adventure deeper into the wastes, however, you'll encounter other players less frequently. No word on specific numbers.”
The world contains five regions with a fully day-night cycle. Dynamic weather is also a feature. Additionally, offline mode won’t be possible.
Thanks PC Gamer.