This volume contains the proceedings of SARA 2000, the fourth Symposium on Abstraction, Reformulations, and Approximation (SARA). The conference was held at Horseshoe Bay Resort and Conference Club, Lake LBJ, Texas, July 26- 29, 2000, just prior to the AAAI 2000 conference in Austin. Previous SARA conferences took place at Jackson Hole in Wyoming (1994), Ville d'Esterel in Quebec (1995), and Asilomar in California (1998). The symposium grewout of a series of workshops on abstraction, approximation, and reformulation that had taken place alongside AAAI since 1989. This year's symposium was...