The balanced reaction
4 Fe + 3 O₂ → 2 Fe₂O₃
says that 4 mol Fe are required to react with 3 mol O₂. If we normalize the coefficients with respect to O₂, we have
4/3 Fe + O₂ → 2/3 Fe₂O₃
so that 4/3 mol ≈ 1.33 mol Fe are needed to react with 1.00 mol O₂.
