Beta build of Forza Horizon 4 accidentally leaked months prior to release due to Windows Store issue
Forza Horizon might not be releasing until October 2, 2018, but due to a major Windows Store error, a full beta build of the game wound up getting leaked. If you purchased the game's Ultimate Edition and had your PC on with enough storage space, the Windows Store began downloading an unfinished version of the game, despite it not releasing for another 4 months from now. Fans have been rooting through the game files, looking for interesting information, and have found a current list of car models that will be available in the full game. You can see those here.
For those wanting to know if they can play the beta build prior to release--no, you cannot, as it's treated as a preloaded game, meaning it cannot be launched fully until release date. It's 90GB in total.
This isn't the first time this has happened to developer Playground Games, either. The previous title, Forza Horizon 3 also had this exact issue, where a dev build of the game accidentally got pushed out from the Windows Store, letting players see plans for upcoming cars and DLC before any official announcement.