\[\boxed{\text{4.\ 182}}\]
\[\textbf{а)}\ 8\ :4 = 2 \cdot 6 = 12 - 2,3 =\]
\[= 9,7 - 0,11 = 9,59\]
\[\textbf{б)}\ 0,6 \cdot 3 = 1,8 + 7,8 =\]
\[= 9,6 - 4 = 5,6\ :7 = 0,8\]
\[\textbf{в)}\ 3\ :\frac{1}{4} = 12 \cdot \frac{1}{12} = 1 - \frac{1}{3} = \frac{2}{3}\]
\[\textbf{г)}\ 1 - \frac{4}{5} = \frac{1}{5} \cdot 2\frac{1}{4} = \frac{1}{5} \cdot \frac{9}{4} =\]
\[= \frac{9}{20}\ :\frac{4}{5} = \frac{9}{20} \cdot \frac{5}{4} = \frac{9}{16}\]