This proceedings contains 47 papers presented at Robotics 2000: The Fourth International Conference and Exposition/Demonstration on Robotics for Challenging Situations and Environments held February 27-March 2, 2000, in Albuquerque, New Mexico. These papers represent the latest advances from the alliance of researchers working on robotic technologies, and those seeking to apply these state-of-the-art technologies in a variety of challenging environments. Paper topics include: space operations, exploration, and construction; robotic excavation and construction; underwater, under-ice, and underground operations; operator control interfaces; human-robotic systems interaction, space medicine, and life support; path planning, navigation and control algorithms; and robotic systems for planetary exploration. These papers cover not only space applications but also applications in terrestrial inspection and maintenance, construction, the nuclear industry, environmental remediation, medicine, and manufacturing. The goal of the Robotics 2000 conference was to foster communication between researchers who might not be aware of each others work, so that knowledge and experience gained in one domain may be applied to another.