The structure of carbon suboxide C3O2 is O = C = C = C = O.
Each of the two terminal oxygen atoms has an oxidation state of −2 and the carbons to which they are attached have an oxidation state of +2.
The central carbon atom in C3O2 has zero oxidation state and the two terminal carbon atoms are present in +2 oxidation state each, whereas the third one is present in zero oxidation state.
Therefore, the average oxidation state of carbon is 4/3.