The central metal hybridisation and the type of complex for the low-spin complex $[\mathrm{Fe}(\mathrm{CN}){}_6]{}^{4-}$ are:

$\mathrm{Fe}{}^{2+}$ is $3d^6$. The strong-field ligand $\mathrm{CN}{}^{-}$ pairs the six electrons in three $3d$ orbitals, leaving two $3d$ orbitals empty. These two $3d$, one $4s$ and three $4p$ orbitals hybridise to give $d^{2}s{p}^3$, an inner-orbital complex.