Key Idea:- In case of high spin complex, Δ0 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 d4 octahedral complex, 
∴ Crystal field stabilisation energy =(−3×0.4+1×0.6)Δ∘=(−1.2+0.6)Δ0=−0.6Δ0