A reactant in an oxidation-reduction reaction loses four electrons when it is oxidized. How many electrons must be gained by the reactant that is reduced?
Clearly, the answer is 4 electrons.
Both mass and charge are conserved in chemical reactions. If 4 electrons are lost from one reagent 4 electrons must be gained by the beast that is reduced, e.g.
It might seem a little bit abstract to use electrons, fundamental particles, as reagents, but a redox reaction in which multiple electron transfer occurs demands this approach if we want to balance the equation straightforwardly. Of course we would not normally assign oxidation numbers to carbon compounds.