Unlike other competing games like Assetto Corsa, Forza Horizon 4 is unfortunately rather limited when it comes to installing mods. However, you can mod it to some extent. To start, make sure you've already played the game beyond the intro and have a save file ready. Then, locate the save file by navigating to your C Drive, head into Users, and open the folder that shows your PC's username. Now, head into AppData, then Local, and finally, the Packages folder. Here, find a folder titled 'Microsoft.SunriseBaseGame_8wekyb3d8bbwe', and open up the SystemAppData folder. Here, you'll now have access to your game's save files, where you can download modded save files from the web to copy them into this folder. These mods can max out your level, add more money, unlock all the cars, and so on.