All of them are dead; 404 or 403 or page not found errors. Including diablo3.com, which they Borged up a few weeks ago and was, until recently, redirecting to the fansite now located at diablofans.com.
If someone wants to try to construct a rumor based on the fact that at least 8 d3-related domain names owned by blizzard are all dead, while their one diablo 4 domain name is redirecting to blizzard's main European page, be my guest. Perhaps bliz knows they'll need the diablo 3 ones pointing to a new site in the immediate future, while they know they'll have no need of diablo4 domain names for years to come?
I tried some others, just for fun. diablo4.com is the only interesting one, since it redirects to http://www.world-of-diablo.com/index.html, which sounds like a fansite, but contains no content other than a title page saying it's been blocked by the entertainment software association. diablo4.net and diabloiv.com are both squated by godaddy.com, and diabloiv.net doesn't exist.