You dont have to re-create them in Horizon. You can kinda cheat and load your vinyl groups from Forza 4 into Horizon. Like this:
Viper
Load your viper in F4 and go to the top view. Save all those top view vinyls as "viper top".
Go to the front bumper save as "Viper front bumper".
Go to rear save as "Viper rear bumper".
Skyline
Load your Skyline in F4 and go to the top view. Save all those top view vinyls as "Skyline top".
Go to the front bumper save as "Skyline front bumper".
Go to rear save as "Skyline rear bumper".
If you have anything on the sides you can save them as "Viper driver side" or "Viper passenger side". Now when you import these to Horizon, you can load "Viper top" to the top of the Viper, side to the side, front to the front, back to the back. Hope that helps.