Ionic Equilibrum

What is the value of pOH if a buffer solution is prepared by mixing equal volumes of $0.4 \mathrm{~M} \mathrm{~NH}_4 \mathrm{OH}$ and $0.5 \mathrm{~M} \mathrm{~NH}_4 \mathrm{Cl}$ solutions. $\left(\mathrm{pK}_{\mathrm{b}}=4.730\right)$