The ionic charge of SO4 is -2. Ionic, or formal, charge is not an actual charge of the chemical, but rather an estimate of electron distribution within a molecule or ion, based on a simplified model.
Formal charge of any atom in a molecule or ion can be calculated from the equation FC = GN – UE – BE. GN stands for atom’s periodic table group number or the number of its valence electrons. UE represents the number of electrons that remain unbound within the molecule or ion. BE stands for the number of atoms' electrons that form covalent bonds. Ionic charge is the sum of formal charges of all the atoms in the ion.