Which of the following salts are composed of isoelectronic cation and anion?

$\left(\mathrm{I}\right) \mathrm{NaCl}$

$\left(\mathrm{II}\right) {\mathrm{BaCl}}_2$

$\left(\mathrm{III}\right) {\mathrm{MgF}}_2$

$\left(\mathrm{IV}\right) \mathrm{CaS}$