Tire age / "dry rotting"

[adamkn]

I have a set of stock Rubicon tires that are ready to be replaced. However, my spare tire has never been used - and I'm wondering if it would be fine to start using it, and just buy 3 new tires, instead of 4. And then I'd just use one of my worn-out tires as a spare.

The spare is probably 3-4 years old (I bought these wheels/tires used, so I'm not totally sure) - and it looks perfectly fine - but I've heard that tires tend to break down over time, from being in the sun, and also from just drying out (the rubber compounds loose their "sticky-ness", so traction isn't as good?).

Thanks for any input!