 In the case of cations with a +4 oxidation state, the only acceptable format for the Roman numeral 4 is IV and not IIII.The Roman numerals in fact show the oxidation number, but in simple ionic compounds (i.e., not metal complexes) this will always equal the ionic charge on the metal.For a simple overview see , for more details see selected pages from IUPAC rules for naming inorganic compounds.