Crystal field splitting energies for octahedral ($\mathrm{\Delta }$₀)  and tetrahedral t ( $\mathrm{\Delta }$_t)  geometries caused by the same ligands are related through the expression.

splitting in tetrahedral complex is 2/3 rd of octahedral complex, 

so for one ligand splitting on octahedral is $\frac{{∆}_0}{6}$

 then for one ligand splitting in tetrahedral is $\left(\frac{2}{3}\right)\frac{{∆}_0}{6}$

 So for 4 ligand, 
                   ${∆}_{\mathrm{t}} =4\times \frac{2}{3}\times \frac{{∆}_0}{6} =\frac{4}{9}{∆}_0$