Magnetic moment of the following is zero

   $$\begin{gathered} \mathrm{Magnetic} \mathrm{moment} = \sqrt{\mathrm{n}(\mathrm{n}+2) }\mathrm{BM} \\ \mathrm{where} \mathrm{n}=\mathrm{number} \mathrm{of} \mathrm{unpaired} \mathrm{electrons} \end{gathered}$$ 

   $$\begin{gathered} \mathrm{Valence} \mathrm{shell} \mathrm{Configuration} \mathrm{of} {\mathrm{Fe}}^{+2} \mathrm{is} 3{\mathrm{d}}^6 4{\mathrm{s}}^0 \\ \mathrm{In} \mathrm{presence} \mathrm{of} \mathrm{strong} \mathrm{ligand} {\mathrm{CN}}^{-}, 6{\mathrm{e}}^{-} \mathrm{in} 3\mathrm{d} \mathrm{are} \mathrm{arranged} \mathrm{as} 3 \mathrm{pairs} \end{gathered}$$

Hence $[\mathrm{Fe}{\left(\mathrm{CN}\right)}_6{]}^{-4}$ has no unpaired electrons & it has zero magnetic moment.