\[\boxed{\mathbf{879}.}\]
\[1)\log_{\frac{1}{4}}64 = \log_{\frac{1}{4}}4^{3} =\]
\[= \log_{\frac{1}{4}}\left( \frac{1}{4} \right)^{- 3} = - 3\]
\[2)\log_{\frac{1}{3}}81 = \log_{\frac{1}{3}}3^{4} =\]
\[= \log_{\frac{1}{3}}\left( \frac{1}{3} \right)^{- 4} = - 4\]
\[3)\log_{\frac{1}{3}}\frac{1}{27} = \log_{\frac{1}{3}}\left( \frac{1}{3} \right)^{3} = 3\]
\[4)\log_{\frac{1}{2}}\frac{1}{64} = \log_{\frac{1}{2}}\left( \frac{1}{2} \right)^{6} = 6\]