The two options are:
(i) If Do < P, the fourth electron enters one of the eg orbitals giving the
configuration 
3
1
2g
g
t
e Ligands for which Do < P are known as weak
field ligands and form high spin complexes (ii) If Do > P, it becomes more energetically favourable for the fourth
electron to occupy a t2g orbital with configuration t2g
4eg
0 Ligands
which produce this effect are known as strong field ligands and
form low spin complexes