Atomic optical lattices offer a method for manipulating neutral atoms accurately and study on several basic problems in physics. We propose a novel scheme to form a controllable four-well optical trap for cold atoms or molecules by using an optical system composed of an amplitude cosine grating and a lens illuminated by a plane light wave. We calculate the intensity distribution of the four-well optical trap, discuss the evolution process of the optical trap from four wells to double wells or single well, and derive the analytical relations between the characteristic parameters of the four-well trap (including geometric parameters, intensity distribution, intensity gradients and their curvatures) and the parameters of the optical lens system. The study shows that this four-well trap can continuously evolve into a double-well or single-well one by changing space frequency of the cosine grating, and the dependence of the distance between the trapping centers of the four wells or two ones on the grating space frequency is obtained.