It would be time to let go the S4 in about 3 years and move on to something else. I do have few cars in mind and S8 is something that def sticks in the back of my mind. I know Audi have a tough time keeping its value "its good/bad" I would love to one day purchased one used. I'm thinking it ought to be in the 70ish range in 3 years?? What is your guess =)

A three year old S8 will probably be worth about 50K +/- 10K depending on options, mileage, condition.