The G8x generation BMW M3 and M4 in Forza are equipped with Sport Brakes by default, meaning there is one Brake Upgrade available, adjustable Race Brakes. Instead of using the awful default Forza model brakes, this upgrade should give the M4 Carbon Ceramic brakes from the Driver’s Package option, which is quite common amongst M3 and M4 buyers in real life.
While yes, many people will say that Carbon Ceramic brakes are worse for the track, the default Forza Race Brake model is… Carbon Ceramic, and they are the only adjustable brakes before Forza Motorsport.