Blizzard has confirmed that the open beta for Diablo 4 will be unleashed in March. It'll be available on both PS5 and PS4, and you'll be able to play through the game's prologue and first act. Based on our own early access to the action RPG back in December, that represents roughly ten hours of content.
Anyway, a pre-order gets you an early peek spread across one weekend, on the 17th March until the 19th March. Then, the beta will be made open to everyone from the 24th March until the 26th March. Not exactly the longest beta period we've ever seen, but it should be enough to give you a taste for the demon-slaying adventure.