A four-line nylon cutter assembly for a mower comprising: an upper case having an insert groove at a central portion thereof, an elastic body in which one surface portion is in close contact with an inner surface of the central portion of the upper case, a spool having a surface on which a nylon cutter is wound and one surface which is in close contact with the other surface portion of the elastic body by pressing, an operating wheel of which one surface portion is coupled to the other surface portion of the spool, and a lower case in which a mounting groove into which the other side portion of the operating wheel is inserted is formed at the center portion and an edge portion is coupled to an edge portion of the upper case, wherein a winding surface of the spool is divided upward/downward into a first winding surface and a second winding surface by a separating plate provided in the center part, the first winding surface has first through-holes facing each other, and the second winding surface has second through-holes facing each other to be perpendicular to the first through-holes at an interval (Δh) vertically; first and second bushings and are installed on coupling parts of the upper case and the lower case, respectively; a pair of first bushings are installed to face the first through-holes of the spool, respectively; a pair of second bushings are installed at left and right sides to face the second through-holes of the spool, respectively, and to be perpendicular to the first bushings; while the center part of a first nylon cutter having a predetermined length is positioned in the first through-holes, the first nylon cutter is wound around the first winding surface many times and then both end parts of the first nylon cutter are exposed to the outside by predetermined lengths through first withdrawing grooves of the first bushings and while the center part of a second nylon cutter having a predetermined length is positioned in the second through-holes 35, the second nylon cutter is wound around the second winding surface many times and then both end parts of the second nylon cutter are exposed to the outside by predetermined lengths through second withdrawing grooves of the second bushings, a guide wall is formed in the spool to have an internal space of a cross structure and connect the first and second through holes (33, 35) facing each other. wherein a winding surface of the spool is divided upward/downward into a first winding surface and a second winding surface by a separating plate provided in the center part, the first winding surface has first through-holes facing each other, and the second winding surface has second through-holes facing each other to be perpendicular to the first through-holes at an interval (Δh) vertically; first and second bushings and are installed on coupling parts of the upper case and the lower case, respectively; a pair of first bushings are installed to face the first through-holes of the spool, respectively; a pair of second bushings are installed at left and right sides to face the second through-holes of the spool, respectively, and to be perpendicular to the first bushings; while the center part of a first nylon cutter having a predetermined length is positioned in the first through-holes, the first nylon cutter is wound around the first winding surface many times and then both end parts of the first nylon cutter are exposed to the outside by predetermined lengths through first withdrawing grooves of the first bushings and while the center part of a second nylon cutter having a predetermined length is positioned in the second through-holes 35, the second nylon cutter is wound around the second winding surface many times and then both end parts of the second nylon cutter are exposed to the outside by predetermined lengths through second withdrawing grooves of the second bushings, a guide wall is formed in the spool to have an internal space of a cross structure and connect the first and second through holes (33, 35) facing each other.