how is SO4 anion formed?

Fig1.                                                                         Fig.2

The sulfur atom in sulfate ion is sp³ hybridized. This is dictated by the fact that it needs to form bonds to four oxygen atoms in a molecule having symmetry. In addition, electrons from the non-bonding p atomic orbitals on oxygen are able to form p bonds with the sulfur atom's empty d orbitals as shown in Figure 2. These p bonds are "interchanged" by resonance, and the net result is similar to that shown in the resonating Lewis structures of Figure 3.

Fig.3