Disclosed is a four-line nylon cutter assembly for a mower, including: 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.