https://www.techradar.com/news/forza-horizon-4-release-date-news-and-trailers

Since the release of the first Forza Horizon game in 2012, Microsoft has established something of a pattern with its Forza releases. When one year sees the release of a Forza Motorsport game, the next will welcome a new Forza Horizon game. 

So, with Forza Motorsport 7 having been released in 2017, there’s a great deal of expectation that 2018 will bring Forza Horizon 4 into the public arena. Not being announced yet, there's very little in the way of solid information about what we'll see in Forza Horizon 4.