Mo. Rev. Stat. § 144.115: Returns kept four years — destroyed, when.

The director of revenue shall keep sales and use tax returns on file in his custody for a period of four years after the sales or use tax becomes due. At the end of four years, the director of revenue may destroy the returns; but no return shall be destroyed until the tax has been paid. The director of revenue, at the end of two years, may cause the returns to be reproduced and destroyed in the manner provided by sections 109.120 to 109.140.