AGES 40 and 4 WHY THEM? The 5'3'' professional jockey not only rode his way into horse racing's Hall of Fame and won both the Kentucky Derby and Belmont Stakes three times apiece, but he and his four-legged friend had the horse sense to make their film debuts alongside Tobey Maguire in Seabiscuit (July 25). WORKS LIKE A HORSE The Idaho native, the youngest jockey to win more than $100 million in career purse money, isn't entirely convinced that he's made it. ''I'm never really satisfied because I always think that I can do a little bit better.'' A SERIOUS HORSE HABIT Guiding thoroughbreds to victory for 20 years put major strain on Stevens' knees, so he retired in 1999. ''I wanted to walk away while I was still fairly healthy.'' But after an 11-month hiatus, Stevens was back in the saddle again. IF HE WEREN'T RACING, HE'D BE... ''Probably touring with some rock & roll band. The drums were my first love.'' CLOTHES HORSE ''For a big race, I try to wear the same clothes that I had on the last time I rode a successful race.'' (And, yes, this rule extends all the way to his, um, jockey shorts.) NEXT Even though Stevens says he's ''totally devoted and focused on acting right now,'' he has no immediate plans to give up racing for good.