Since there is debate about releases…
While we know Shannon Loftis, general manager for Microsoft Studios Publishing, initially said at Gamescom last year after the AOE4 reveal that they “wouldn’t hold AOE4 for the definitive editions” if the case may be, Adam Isgreen however, Microsoft Studios Publishing director for AOE4 (“the guy on the Microsoft side making sure AOE4 is going to be an AOE game” to quote him exactly) later said otherwise in an interview taken after the release of AOE1: DE in February 2018, quite clearly letting us know the definitive editions are going to come first, referring to them as a “build up” for the latter AOE4 and that “people should love all definitive editions first”:
Interviewer: Age of Empires was a massive brand in the late 90s and early 00s, but these days it isn’t the behemoth it once was. Is there a pressure for this game to do well, and help reintroduce the brand, ahead of Age of Empires 4’s release?
Adam Isgreen: Most certainly. Sometimes I don’t think companies treat their IP with the respect that they deserve or they should. I’ve been really delighted that we’ve been given permission and support to go and make the Age of Empires games the way we want to make them and make them as good as they can be for the fans and support all the features that the fans would love. There’s a pressure, though. We want Age of Empires 4 to be an amazing game and the team at Relic has been doing some amazing work on it. I’m very happy with how it’s going now. The message we’re sending with all of the Age of Empires games as we go through them and do these Definitive Editions is that we really care about this franchise and what makes an Age of Empires game an Age of Empires game. This is all part of not only showing how great these games were, but it is a build up to a new game. We have a lot of respect for this IP, so hopefully, when Age 4 comes out, people will say they’ve loved all three Definitive Editions and will jump in even though they don’t know much about it. There are a lot of people who still play the Age games on a daily basis, a lot more than people realise. Maybe because it’s not in the limelight of the press all the time. It’s a big number.
This should also mean that AOE4 is still along way from being released, definitely not 2018.
Source: How and why Microsoft updated Age of Empires for the upcoming Definitive Edition | PC Games Insider
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Also, related to the definitive editions, an interesting reveal from the same Adam Isgreen, if you are interested, regarding Steam cross-play:
We’re not leaving Age of Empires fans on Steam behind – we plan to release new content and features – like cross-network play – to let Steam players come along on the Age of Empires journey with us.
This can be quite big, because not only does it mean they have plans for cross-play between HD and DE, but it should also mean that AOE2: DE in particular will include all the new expansions as well (needed in order to work with Steam version). That a lot of work changing all that. I think AOE2: DE is far from release too.
Source: Inter-review: To war with the "new" Age of Empires and Microsoft's Adam Isgreen - Neowin