Adjustable linkages can generate different paths with a simple adjustment of the fixed pivot position of the driven side-link. Limited work has been done in the area of synthesis of four-bar linkages for continuous path generation, especially of adjustable four-bar linkages. There are two kinds of adjustable four-bar linkages with fully rotatable driving side-links, adjustable crank-rocker and adjustable drag-link linkages. The path flexibility of the two adjustable linkages is analysed. The optimal synthesis model is set up based on the link-length structural error of the driven side-link introduced in this paper, which can effectively reflect the overall difference between the desired and the generated paths, can avoid the difficulty of selecting corresponding comparison points on the two paths, and can be calculated conveniently. A modified genetic algorithm is employed to obtain the global optimal solution. The results of two optimal synthesis examples verify the effectiveness of the proposed method.