Blizzard is interested in cross play for Diablo IV, but no promises yet.
As reported by Telegraph (via VG247), Executive Producer Allen Adham is excited about cross play,
“We’re very excited about cross-play. There are technical details and details to work through with the first parties, but it’s our goal to get to cross-play.”
That’s hardly a confirmation, but cross play has become more and more mainstream in recent years with the latest Call of Duty featuring it. We’ll have to wait and see.