In $[Fe(CN)_6]^{4-}$, iron is in the +2 oxidation state ($3d^6$). Cyanide is a strong field ligand, causing the d-electrons to pair up, leaving two empty 3d orbitals. These two 3d, one 4s, and three 4p orbitals hybridize to form six $d^2sp^3$ hybrid orbitals.