If you were talking 5-10 year old tires, I'd replace the spare. That is what I did with my 10 year old tires when I bought my used trailer. Being that it will only be 3.5 years old, I'd keep it and just replace the 4. I'm not even sure I'd replace any of them at 3.5 years old unless you are really suspect of the tires. I have Maxxis on mine now and have had no issues since I put them on 4 years ago and unfortunately they were 3 years old when they were installed "new" so they are really 7 years from date of manufacturer. I'll replace them for next year at which point they will probably have about 10k miles on them before we take a long trip.
