Looks like you can now sign-up for Diablo 4 beta?
Here is the link to sign up: 
https://diablo.blizzard.com/en-us/signup

There are also other stuff on that site about Diablo 4:
The 3 classes: https://diablo.blizzard.com/en-us/classes
Game Play: https://diablo.blizzard.com/en-us/gameplay
Diablo 4 world: https://diablo.blizzard.com/en-us/world
The story: https://diablo.blizzard.com/en-us/story
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