In complex [Ni(CN)4]2- does nickel acquire negative charge, as 4 CN ligands gave 4 pairs of electrons. Does the bond became ionic as there is complete transfer of electrons?????

No, the bonds are not ionic,they are coordinate bonds.In this complex Ni does not acquire negative charge it carries +2charge as 4 CN^- ion create 4 coordinate bonds with Ni2+ ion.Please clear your concept about coordinate bonds, it will help you.