For the first time in the series Forza Horizon 4 is a shared world game. It’s a big change, and one which Playground Games creative director Ralph Fulton describes as a “huge paradigm shift.” It’s also a potentially a concerning one for some but, as it’s still optional, it probably shouldn’t be.
First and foremost, the nature of Forza Horizon 4’s shared world changes the other racers you’ll see while cruising around the open world. In Forza Horizon 2 and 3, the cars you saw hooning around the environment were what the Forza series dubs Drivatars; AI powered driver profiles drawn from your friends list and beyond. In Forza Horizon 4 we’ll still race against Drivatars when we start an event, but the cars you’ll encounter in free roam will now be real people within a 72-player server. The reason Playground Games is doing this is because the team believes that real players will exhibit more interesting, more unpredictable, and more fun behaviour than they could ever program into Drivatars.
Source: IGN.com Forza Horizon 4's Shared World Explained