Step 1:

The degenerate d-orbitals (in a spherical field environment) split into two levels i.e., $e_g$ and $t_{2g}$ in the presence of ligands.
The splitting of the degenerate levels due to the presence of ligands is called the crystal-field splitting while the energy difference between the two levels ($e_g$ and $t_{2g}$) is called the crystal-field splitting energy. It is denoted by ∆o.

Step 2:

After the orbitals have split, the filling of the electrons takes place. After 1 electron (each) has been filled in the three $t_{2g}$ orbitals, the filling of the fourth electron takes place in two ways.

It can enter the $e_g$ orbital (giving rise to ${t_{2g}}^3 {e_g}^1$ like electronic configuration) or the pairing of the electrons can take place in the $t_{2g}$ orbitals (giving rise to ${t_{2g}}^4 {e_g}^0$ like electronic configuration).

If the ∆o value of a ligand is less than the pairing energy (P), then the electrons enter the $e_g$ orbital. On the other hand, if the ∆o value of a ligand is more than the pairing energy (P), then the electrons enter the $t_{2g}$ orbital.