PureDiablo has a video from the IGN Fan Fest where Blizzard's Joe Shely announces dates for two beta testing weekends for Diablo IV. Those who pre-purchase the action/RPG sequel on any platform will get an Early Access weekend with the beta beginning on March 17th. Everyone else can get in on an Open Beta the next weekend, beginning on March 24th. There are no plans for a wipe between the two weekends, but all progression will be lost upon the launch of the full game. It is noted that unlike some betas that come a week before a game's launch, the schedule here will allow the developers to incorporate feedback into the final game before its arrival on June 6th. Here are excerpts where the Diablo IV Game Director outlines the schedule, and a bit on what the beta will show us:

Remember, of course, that the game itself is coming out on June 6th. And you're going to be able to play the beta on March 17th through March 19th, so that weekend, if you've preordered… Now everyone, everyone in the world will be able to play it on March 24th through March 26th, the following weekend.
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And the beta itself, it's quite extensive, it's quite a significant look at the game. It is limited by that period of time, the servers will shut down at the end of that time. But there's no time-limit within the game. You can level all the way up to 25, which is quarter of the way to the level cap of 100 in the full game. You'll be able to play all the way through the Prologue. You'll be able to play through Act I, which takes place in Fractured Peaks, one of our five areas in the game. And you'll be able to play through all the campaign and side-quests that exist in that area. So there's quite a lot to check out.