I am sure that literally everyone expects a Forza Horizon 4 to release this year- Microsoft has not only kept the Forza series annual, switching out between the Motorsport and the Horizon games on a yearly basis, but Forza Horizon 3 represents one of their most successful games to boot, too.
So it should come as absolutely no surprise to anyone that Forza Horizon 4 has indeed been rated for release in Brazil, indicating that it is coming- the game has been rated for Xbox One and PC, indicating yet again that this will be a Play Anywhere game. Sadly, the listing does not give us a look at who the developer might be- something that has been a bit in question given the general rumors that Playground is working on a new action RPG.
Regardless of that, a Forza Horizon 4 should definitely be real, and we will probably see it some time later this year. Hopefully it gets an announcement at E3.