A single molecule of hemoglobin can bind to 4 molecules of oxygen gas. However, hemoglobin has a greater affinity for carbon monoxide than oxygen. Therefore, an excess of carbon monoxide in the presence of oxygenated hemoglobin will result in the displacement of each oxygen atom for a carbon monoxide atom.
Hb(O2)4 (aq) + 4 CO(g) --> Hb(CO)4 (aq) + 4 O2(g)
With an excess of carbon monoxide, it is safe to assume that each oxygen molecule will be displaced with a carbon monoxide molecule. Therefore, if we have 4.5 moles of oxygenated hemoglobin (Hb(O2)4), all 4.5 moles of the species will release oxygen and bind to carbon monoxide.
