What is the oxidation number for s in s2o4?

The oxidation number of O is - 2 and there's 4 of them so the total is - 8. Since it's a molecule the sum of the oxidation numbers of the atoms must equal 0. If 4 oxygen atoms have an oxidation number of - 8 then the two S atoms must is + 8 to equal 0 so the oxidation number of the S is 8/2 = + 4