Earlier today, Blizzard announced that they will open up a Closed Beta for Diablo IV soon. Unlike other tests before, the development team explicitly stated that they are going to invite participants who have recently spent significant amounts of time playing the end-game experiences of Diablo II: Resurrected and Diablo III! You must sign up by October 11, and if you haven’t received a mail invite by November 18, then you’re not selected.
Here is how you sign up for Diablo IV’s Closed Beta: