If the reactants of a reaction have one s and four o atoms, what will the products have?

            The products will also have one S and four O atoms.

Explanation:
                   According to Law of Conservation of Matter, " Matter can neither be created nor destroyed, but it can be changed from one form into another".
                   So, in given scenario the number of atoms in reactants and products will also remain the same, but their chemical composition can vary. They might have no similarity in chemical composition but their numbers will remain the same.
                   The reaction might be the decomposition of sulfuric acid,, 

                                        H₂SO₄     →     SO₃  +  H₂O

In above case number of S and O on both sides are same, but chemical composition has changed.