Blizzard has not yet announced the theme for Season 4, but an initial Public Test Realm release will feature tweaks and overhauls to certain mechanics before a full reveal. Changes include a rework of the Codex of Power where now every single Legendary Aspect will be available, rather than a select few. Tempering is also being added which lets you attach stat-boosting or effect-applying Affixes to gear items. Masterworking will be for late game gear refinement and that includes Unique tier, which otherwise can't be Tempered or Affixed.

Diablo IV
Blizzard Entertainment
Action
RPG
Dungeon Crawler
Single-player
Multiplayer
Season Pass
Battle Pass
In-App Purchases
Microtransactions