According to crystal field theory (CFT):

$d^4$ to $d^7$ co-ordination entities are more stable for strong field as compared to weak field cases

Tetrahedral complexes have sufficiently large splitting energy to force pairing and therefore, high spin configuration are rarely observed.

Order of increasing field strength of ligands is: ${\mathrm{F}}^{-}>{\mathrm{H}}_2\mathrm{O}>{\mathrm{NH}}_3$

If pairing energy exceeds the magnitude of crystal field splitting then pairing occurs