A minimal wheelbase length of a four-wheeled walker was analyzed for prevention of tipping on slopes surfaces by a theoretical analysis of a two-dimensional mechanical model in a sagittal plane. The minimal wheelbase length was obtained from the lower limit of the safety condition derived from the mechanical model. It depended on some design and environmental factors. The normal force applied at the handgrip and the acceleration of walker movement were human factors for ergonomic design of the four-wheeled walker. They were measured by force transducers and acceleration sensors with the use of a commercial walker for validation of the mechanical model. The subjects were ten aged males and ten aged females. The minimal wheelbase length was lengthened with a weaker normal force applied at the handgrip during uphill movement. However, it was shortened during downhill movement. Female subjects should use a walker with a longer wheelbase because they produced a weaker normal force at the handgrip than male subjects. The walker accelerated more and decelerated more on the downhill, but a longer wheelbase was not obtained by a larger acceleration during steeper downhill movement. The higher handgrip led to a longer wheelbase obviously, and the mass of the walker was not a critical design factor. The minimal wheelbase length should be longer on a high friction road, but it was not easy for users to handle the walker. The minimal wheelbase length was shorter than the actual wheelbase length of the commercial walkers. Many commercial walkers were designed that their handgrip was positioned near the rear wheels. This arrangement was a valid design from an ergonomic point of view because it maintained a margin of walker stability for prevention of tipping.