Number of unpaired electrons in $[\mathrm{Fe}{\left(\mathrm{CN}\right)}_6{]}^{4-}$ ion
 

[Fe(CN)₆]⁴– has zero unpaired electrons.

[Fef₆]^4- contains 4 unpaired electrons.

F^-  = weak field ligand

For an atom that is going to be in a complex ion to have the most stable configuration, it will have its highest number of unpaired electrons. 

In the case of [Fe(CN)₆]s and any other metal ion complex, this would be 6.  

The complex ion is called a hexacoordinate complex ion or hexacoordinated complex ion.