The term "four-sided" refers to an object or shape that has four sides. There are numerous synonyms for this word, including quadrilateral, tetragon, rectilinear, rectangular, four-cornered, and four-edged. Each of these words describes an object or shape that has four sides and can be used interchangeably with "four-sided" in most contexts. For example, a square is a quadrilateral, a rhombus is a tetragon, and a rectangle is both rectilinear and rectangular. Using synonyms such as these can help to add variety and nuance to your writing and make your language more precise.