Since splitting in tetrahedral complex is $\frac{2}{3}$ rd of octahedral complex ,so for one legand splitting in $OH =\frac{\Delta_{0}}{6}$, then for one legend splitting in tetrahedral is $\frac{2}{3}\left(\frac{\Delta_{0}}{6}\right)$, so for 4 legand, $\Delta t =4 \times \frac{2}{3} \times \frac{\Delta_{0}}{6}$
$=\frac{4}{9} \Delta_{0}$