The formula of pentaphosphorus tetraoxide is P₅O₄
As per the convention of naming chemical compounds - the Prefix names are used for naming covalent compounds.
The prefix 'penta' means '5', so we use 5 as subscript for P.
The prefix 'tetra' means '4', so we use 4 as subscript for O.
Therefore the formula of pentaphosphorus tetraoxide is P₅O₄