This article presents WAVE, a four-bar linkage-based stair-climbing robot. WAVE is designed to be a last mile delivery robot that can deliver goods on stairways and flat ground. When the cranks comprising WAVE undergo rotation, the body and legs rotate parallel to each other. The rotating body and legs climb the stairs alternately. WAVE has a right-angled groove tailored to the shape of the stairs at the bottom so that it can step stably on the stairs. To prevent the robot from tilting quickly when exiting the stairs, the groove was designed to divide the angle of the robot into three. In-wheel motors are attached to the bottom of the WAVE so that it moves using the wheels on flat ground. Also, as a result of the dynamic analysis, we obtained the motor torque and friction coefficient required to climb stairs. The length of the crank, which is a design variable, was determined via mechanical analysis. The first experiment was performed on the test bench to ensure that the WAVE worked as intended. In addition, through field tests, it was confirmed that WAVE can move upstairs and downstairs on stairs of various sizes.