What is the oxidation number of carbon in CO2

Since oxygen is in group 6 on the periodic table, its valency is  

2 −

and hence so is its oxidation number.

But there are 2 oxygen atoms for each carbon atom so the total negative charge due the oxygen is  

4 −

But since a molecule must have a net charge of zero in the free form, it implies that the oxidation number of the carbon atom must then be  

+ 4