From Diablo 4 Wiki

All Blizzard games go through the beta test process with the public and Diablo 4 is no exception.

Pre-Registration

Blizzard launched their Diablo 4 beta test pre-registration opt-in for players on 11 june 2022. Potential testers could join the lottery to be picked via the opt-in link.

Pre-announcement Promotion

On 11 July 2022, Blizzard launched a marketing campaign called Hell's Ink. The campaign invited fans to attend tattoo parlours around the world to receive a Diablo themed tattoo.

Hell's Ink Promotion locations
Hell's Ink Promotion locations

One of the rewards for receiving a tattoo was access to the Diablo 4 beta test.

Tattoo reward
Tattoo reward

Launcher Update

On 21 July 2022, the BattleNet launcher received an update which indicated that they were now preparing for a Diablo 4 Beta test. The addition of Fenris assets to the launcher in the file changes was a key sign Blizzard was moving toward a test.

Following the BattleNet launcher update discovery, Blizzard's Rod Fergusson dampened expectations for a quick start to the Diablo 4 beta and posted:

Hey all, I’m loving all the energy and excitement about a Diablo IV Beta showing up on http://Battle.net but you should know that this is an internal only test. Sorry! We will have more public testing in the future as we continue down the path to shipping in 2023!

When will the Diablo 4 Beta Test Begin?

Following the Hell's Ink promotional campaign and the BattleNet launcher update, there is speculation that the the Diablo 4 beta test could begin as early as September 2022, and most likely before the holiday season.

How Long will the Beta Test run?

Based on previous tests for games such as World of Warcraft, the Diablo beta test could be lengthy, possibly as long as six months.

Due to the fact the game is set in an open world and is a lot more complex than previous Diablo titles, Blizzard is going to need a longer than usual testing period for a Diablo product.