In this paper, we propose a novel scheme to build a controllable four-well optical trap for cold atoms or molecules by using an optical system of a binary π-phase plate and a lens illuminated by a plane light wave. We calculate the intensity distribution of the four-well optical trap, and discuss the evolution process of the optical trap from four wells to two wells or form four wells to a single well, and derive some 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. Our study shows that our four-well trap can be continuously changed into a double-well one or a single-well one by moving the π-phase plate, and the relation between the distance of the trapping centers of the four wells or two wells and the moving distance of the π-phase plate is obtained by data fitting. So our four-well trap scheme has important and wide potential aplications in the fields of ultracold atomic physics, cold molecular physics, atom optics, molecule optics and quantum optics,and even in the fields of quantum calculation and information processing, and so on.