https://www.polygon.com/2018/6/24/17498946/forza-horizon-4-leaks-car-list-vehicles

A Forza fan on PC who preordered the Ultimate Edition of Forza Horizon 4, yesterday discovered the game downloading through the Windows Store early. Examining the game files, they discovered a roster of more than 450 cars.

It’s important to note that this may not (and considering DLC, probably is not) the full vehicle lineup for Playground Games’ open-world racer, just announced at E3 2018. The game is still more than three months from launch and vehicles could be added in or held back. (The lineup changed between a similar leak and the launch of Forza Horizon 3 in September 2016). The original poster said there are some manufacturers who will appear that aren’t in this list, for example.