The four-bar linkage mechanism is widely utilized in robotic arms due to its structural simplicity and motion stability. However, its rotational motion often demands significant torque, leading to increased energy consumption. Attaching elastic elements such as rubber bands to the linkage can substantially reduce the required torque, thereby enhancing energy efficiency. Despite this potential, current design approaches largely rely on empirical methods and trial-and-error, lacking systematic optimization strategies. This study presents an intuitive design program for rubber-band-assisted four-bar linkages, aimed at improving both design efficiency and accuracy. A numerical torque model was developed to analyze the forces exerted by rubber bands on the linkage, and nonlinear corrections were applied based on experimental data to enhance model validity. A comprehensive performance index was constructed to evaluate system effectiveness, and a genetic algorithm was employed to identify optimal parameter combinations. Results indicate that the optimal placement of rubber bands can offset approximately 98% of the work performed by the linkage, demonstrating significant energy-saving potential. Furthermore, the design program was applied to the development of a passive hip flexion exoskeleton. The generated assistive torque curve was tailored to match hip joint gait requirements, effectively reducing muscular load and improving walking efficiency. Simulation results revealed an energy saving of approximately 90%, with assistive torque contributing during 6% of the gait cycle. These findings highlight the potential of the proposed system as a low-cost, power-free exoskeleton design tool.