It's not Warcraft IV, but it'll have to do: Today Blizzard announced it's importing over three thousand Warcraft III assets into the StarCraft II mod tools. Everything from the character models to the audio and UI are available, meaning someone could conceivably recreate basically all of Warcraft III in the StarCraft II engine.
Someone like... Blizzard, maybe?
Will we see a Warcraft III HD anytime soon? I don't know, but it's at least a possibility. And in the meantime we'll undoubtedly see modders recreating some of their favorite levels... or just remaking Defense of the Ancients a thousand times over.
"Once these assets become available to the public, we are going to spotlight the best content in what we are calling, 'Memories of War,'" writes Blizzard. "Whether it's from the first opening of the dark portal in Warcraft: Orcs and Humans to Grommash leading the Iron Horde in Warlords of Draenor, we want to see it recreated in the StarCraft II editor."
It's a smart move--not only is there a ton of Warcraft III nostalgia floating around, but there's a ton of nostalgia floating around for Warcraft III's mod community in particular. There's a reason Dota emerged from that crucible. It's one of those rare games (like Counter-Strike 1.6) that was surrounded by an outpouring of creativity and community goodwill.
And for all you Warcraft fans reading this and thinking "Oh damn, I don't have StarCraft II," remember: The StarCraft II arcade (which is where all the custom maps reside) is entirely free-to-play. Zug zug.