I heard somewhere that the limit on tires is about 6 yrs, whether you use them or not. If they are filled with nitrogen, they will supposedly last longer. Besides UV light, pollution, weather, etc. (exterior factors), some studies say tires start to break down from the inside due to moisture in the air. I'd get a set of 4 new tires and keep the spare as an emergency spare figuring you'll have to toss it next time or get 5 new tires and do a 5 tire rotation.
My spare is still the original dunlop. It's evident that the PO never mounted it, as I still found the round piece of paper and some plastic cap attached to the wheel where the wheel mounts to the cable attachment. My plan is to run the current bridgestone HT's down, then replace all 5 tires and do a 5 tire rotation. This way, at least all your tires are the same age and have the same wear.