They lose the natural oil that keeps them soft. Think of the rubber molecules like fresh, wet spaghetti, then dried out spaghetti. At four years old and maybe 30 passes, i may have lost .02 60 ft. It is tuff to through away what looks like a good tire, but IMHO it is time.
One other thought. Some guys switch out trailer tires sooner than that, and hopefully they aren't going as fast😀