Forza: Horizon is a spin-off from the classic Forza Motorsport title scheme, focusing on an open-world racing. The first game was set in Colorado and released in 2012 exclusively on Xbox 360. Forza Horizon 2 came out two years later entering the next generation of Xbox consoles with beautiful vistas of Southern France and Northern Italy. The third game was set in Australia and debuted two years ago.
As you can see, it makes sense to expect to play Forza Horizon 4 in 2018 and it looks like that's exactly what's going to happen. Fernanda Delgado, the head of Xbox in Spain and Portugal, has confirmed that Forza Horizon 4 will be announced at E3 in June. "There have been many speculations about where the next Forza Horizon will be and that is what we will tell later at E3," she told GameReactor.
The game will be allegedly set in Japan which would be a nice change of pace after the brown and sunny world of Australia. It's unclear who's working on Forza Horizon 4 but considering that the first three games were developed by British Playground Games which is apparently working on the next Fable, it seems that the fourth chapter of the Horizon series is being developed by a different studio.