Part 3 (of four): Time is the theme grouping the four artists profiled here. Among them: Martin Puryear, who's known for his large installations; Paul Pfeiffer, whose installations and video projects play off pop culture and sports; Vija Celmins, who specializes in natural forms; and Tim Hawkinson, whose installations have been as big as a football field. Choreographer Merce Cunningham hosts.