Now in your question the given ion S4O6^2- is the tetrathionate ion. Each oxygen atom has an oxidation state of -2. Since the total oxidation state for tetrathionate is -2, and there are six oxygens, each with an oxidation state of -2 for a total of -12, the sum of the sulfur oxidation states has to be equal to -2 - (-12) = +10. Since there are four sulfur atoms, their AVERAGE oxidation state is 10 / 4 = +5/2.