(2) Sodium propionate is a food preservative with the chemical formula of C3H5O2Na, which consists of four elements: carbon, hydrogen, oxygen and sodium;
(3) Small-package pastry foods are often packaged by air, and nitrogen is a commonly used filling gas, mainly considering that the chemical properties of nitrogen are stable and it is not easy to react with other substances;
(4) Iron reacts with oxygen and water to generate iron hydroxide, with known reactants being iron and oxygen and water and products being iron hydroxide; According to the law of conservation of mass, the chemical equation is written correctly: 4Fe+3O2+6H2O=4Fe(OH)3.
So the answer is: (1) O2; (2)4; (3) Nitrogen is chemically inert; (4)4Fe+3 O2+6H2O = 4Fe(OH)3；