Age of Empires IV is in development by Relic Entertainment, creators of the Company of Heroes series and other beloved strategy games.
Other than its existence, and the intention to publish the game exclusively on Windows 10, publisher Microsoft Studios revealed almost nothing about the project. Judging by the animated concept art in the trailer, Age IV won't take the series to a singular new era. Instead, a variety of armies from previous games in the series are featured in the footage: Native Americans, British Redcoats, Roman legionnaires, Japanese samurai, and others. It's unclear whether these are the units or factions we'll control in Age of Empires IV. Hopefully they won't take the form of individually purchasable armies, as Relic did with Sega's Company of Heroes 2.
Relic has added a few words on its own website about the announcement.
"[We] couldn’t be more proud to be a part of the legacy," a blog post reads. "We can’t wait to tap into our strengths as a studio to fuse historical context with deep strategic gameplay, and to bring this franchise back to the forefront of gaming and into the hands of its beloved community."
Age of Empires IV will be Relic's first game produced for a publisher other than Sega, its owner, since it was purchased by auction in the selling off of THQ's assets in 2013. Speculating, it's likely that Microsoft paid Sega for the right to work with Relic, as it probably did for Creative Assembly, the Sega-owned studio that developed Halo Wars 2.
Age IV is a separate project to Age of Empires: Definitive Edition, the remaster of the original game that was announced at the PC Gaming Show at E3. As part of the announcement, we also learned that remastered versions of Age of Empires 2 and 3 are in the works.
In 2009 Microsoft disbanded Ensemble Studios, the original creators of Age of Empires, eight years after acquiring it in 2001.
Hat tip to Bill Gates, who presumably got this whole thing rolling after answering this question in a Reddit AMA in March 2016: