Yeast Longevity

This was baked yesterday for a friend so I won't get a crumb photo. It had eleven hours fermentation at a cool room temperature. What surprised me was how viable the yeast is (saf-instant) since it is about four years past the use-by date. It is kept in the refrigerator. This loaf has 600 grams of flour (all-purpose, whole wheat, and rye) and about 1/8 teaspoon of yeast.