I've tried modifying all of the tier 4 cars but all I can change are the visual looks of the car. When I try to upgrade the car it says "there are no upgrades available for this car."
But, when I race any of the Tier 4 "Works Restricted" races I see plenty of Tier 4 Works upgraded cars being driven by the AI. Especially the Pagani Zonda F. While I'm stuck with a Tier 3 Works upgraded car.
Is there no way for us (the players) to be able to have upgraded Tier 4 Super Cars?
No clue if there's anyway to get it for the player (Via mods/tweaks, or otherwise), but the AI is more than known to Cheat on many occasions.
Rather it be using modified Tier-4 cars, or just in general using vehicles that are outside the restrictions for any race (Ie, using Tier-3/4 cars in a Tier-2 restricted race). Assuming it's not a (major) oversight/flaw in the game design, I can only guess that it's there for a bit of balance to make the AI more challenging in races.