This paper describes a novel four-limbed robot having high mobility capability in extreme environments. At disaster sites, there are various types of environments where a robot must move such as rough terrain with possibility of collapse, narrow places, stairs, vertical ladders and so forth. In this paper, first we categorized extreme environments based on three indexes: unevenness, narrowness, and inclination. To move in such extreme environments, we proposed a four-limbed robot having various locomotion styles such as bipedal/quadrupedal walking, crawling and ladder climbing. The main contribution of this paper is the concept and hardware design of the four-limbed robot. We developed a prototype of the four-limbed robot having commonly structured limbs. The number of DoF for the whole body is 29, with 7-DoFs in each limb and 1-DoF in the trunk. The robot weight is 110 kg, and the height is 1,290 mm when standing on two legs. The end-effector has hook-like shape. Verification of the prototype robot is conducted through simulations and experiments.