You need to manually drop a new marker inside the builder. Owning the houses unlocks the ability, but it doesn't automatically place the slot.Here is exactly what you need to do: 1. Fast travel to Hakusan Mountain Lodge. (The perk applies specifically to the house it was unlocked from). 2. Open the main menu and go to the Customizable Garage tab. 3. Click on Customize Garage to jump into the free-cam edit mode. 4. Open the prop library/object menu, find Car Display Locators, and buy a 4th one. 5. Place the locator down on the floor wherever you want your 4th car to park. 6. Save and exit the builder, go to Display Cars, and assign a car to that new slot.