In case of high spin complex, $\Delta_{0}$ is small. Thus, the energy required to pair up the fourth electron with the electrons of lower energy d-orbitals would be higher than that required to place the electrons in the higher d-orbital. Thus, pairing does not occur.
For high spin $d^{4}$ octahedral complex,
$=(-3 \times 0.4+1 \times 0.6) \Delta_{0}$
$=(-1.2+0.6) \Delta_{0}=-0.6 \Delta_{0}$
