Microsoft ran a special Age of Empires stream earlier today, which ended with the announcement of Age of Empires IV. Details are somewhat scarce at the moment, but one thing that is known is the developer: Relic (of Homeworld, Company of Heroes fame).
The trailer (below) takes a concept art driven trip through human history and makes the expected noises about discovery, conquest and the like. It ends with a Windows 10 logo, which would suggest that, like the upcoming Definitive Edition, Age of Empires IV will be Windows 10 only. That also means it’s likely to be stuck on the Windows store. That may not be the case though, and games don’t always remain unique to the Windows 10 store forever (however, ones published by Microsoft generally do).
There’s no release date or really any further information just yet.