There are no access requirements, so simply keep an eye on the relevant digital storefront for your platform and download the client when it becomes available. The only way to access the Diablo 4 beta now is to wait until May 12-14, when the next (and likely final) Open Beta will be available for everyone who knows how to download the Diablo 4 beta. However, that ran from March 17-20 which has now passed. It was also possible to access the Diablo 4 beta by pre-ordering the game, with various different versions available including the Standard, Deluxe, and Ultimate Editions, and each provides a variety of rewards within Diablo 3 and 4 including the Early Access Beta. Previously it was possible to sign up to access the Diablo 4 beta via the official website, at least according to a Blizzard support post regarding the Alpha and Beta testing phases of the game published in 2022, but that option is now no longer available.