I have a set of tubular Continental Sprinter Gatorskin (28" x 22mm) that have been sitting in the box on the floor for close to four years. They look fine, but I'm worried that they won't last long if I mount them. Should I get new tires?
Some extra information: I have an iron-distance triathlon soon and my rear tire popped today.
The shelf life is going to be different depending on the quality and composition of the specific tire, and the storage conditions. In general though, you should be able to see if a tire has gone over or not: if the rubber is still soft and pliable, it should be fine to use.
I'd guess that your tires are still okay, but without seeing the tire i can't give you better than a guess.
Back in the day, when everyone used tubulars, serious riders would age them intentionally. There's no science to back this up, and no measurable benefit, but apparently no serious detriment either, as long as they're still in good shape. If the sidewalls or tread are cracking, they're no good.