$PH=4/7\Rightarrow \left[ {{H}^{+}} \right]={{10}^{-4/7}}={{10}^{-5}}\times {{10}^{0/3}}=2\times {{10}^{-5}}$ $\Rightarrow \left[ O{{H}^{-}} \right]\times \left[ {{H}^{+}} \right]={{10}^{-14}}\Rightarrow \left[ O{{H}^{-}} \right]=\frac{{{10}^{-14}}}{2\times {{10}^{-5}}}$  $\frac{\left[ {{H}^{+}} \right]}{\left[ O{{H}^{-}} \right]}=\frac{\frac{2\times {{10}^{-5}}}{1}}{\frac{{{10}^{-14}}}{2\times {{10}^{-5}}}}=4\times {{10}^{4}}$