If you purchased the Digital Deluxe Edition or Ultimate Edition of Forza Horizon 4 then you must be waiting to drive all the new DLC cars that came with both editions. If you start the game and do not find any sort of DLC then do not worry, the game is meant this way. You must first finish the prologue and reach the autumn season to unlock more features of the game.
You will get your first house before the autumn season starts and once autumn season begins, you will need to finish the first dirt series race. Once the race is over, the game will unlock all features for you such as Online, Autoshow, Auction House etc. Once everything is unlocked, you will start getting notifications on your screen about different things.
These notifications will include your VIP membership bonuses, new cars added to the garage and a free house gifted to you if you have the VIP. Once these notifications are gone, you can simple head to either one of your houses or the Horizon Festival Site and access your garage. Here go to Autoshow and the DLC Cars tab to view all cars have been given to you free of cost in Best Bond Car Pack or the Formula D Car Pack.
All DLC cars are free to purchase one time. You can easily all DLC cars to your garage and then pick whatever car you want to take it out for a spin. However, if you plan on buying a DLC again after adding it to your garage one time, you will need to pay CR for that. The game also comes with a Car Pass, which will further add more cars to the game in the future.
If you have purchased the Ultimate Edition, you can jump in the game right now with Early Access however if you have the Standard Edition then your game will unlock on October 2. Forza Horizon 4 is available on Xbox One and PC.