According to the Brazilian Classification Rating Board that deals with the evaluation of video games, Forza Horizon 4 has been listed in their database and shows the release year of 2018.
No other details are mentioned except for the release year and the name of the game. This is definitely a sort of confirmation that the game will be released in 2018 as a playable version must have been made available for testing considering that it has made it to the rating phase.
Rumours stated that the game will be set in Japan and will be developed by Playground Games, under the supervision of Turn 10 studios. Rumours also state that the game has a release window of early September. But, nothing has been announced officially, and these are just rumors.