A method for preparing a tetrapolymer includes preparing a suspension including an aromatic vinyl-based monomer, an unsaturated nitrile-based monomer, an N-substituted maleimide-based monomer, an α-alkyl styrene-based monomer, and an inorganic dispersing agent, and polymerizing the suspension. The method for preparing a tetrapolymer can be useful in lowering a reaction temperature using suspension polymerization, and reducing a reaction time, thereby maximizing productivity. A tetrapolymer prepared by the method is also provided.