\[\boxed{\mathbf{4.}\mathbf{154}}\]
\[\textbf{а)}\ 2\ :4 = 0,5 \cdot 3 = 1,5 - 1,2 =\]
\[= 0,3\ :0,1 = 3\]
\[\textbf{б)}\ 6 \cdot 0,6 = 3,6 + 1,2 =\]
\[= 4,8\ :0,4 = 12 \cdot 0,3 = 3,6\]
\[\textbf{в)}\ \frac{1}{3} - \frac{1}{6} = \frac{1}{6}\ :\frac{1}{2} = \frac{1}{3} \cdot \frac{3}{7} = \frac{1}{7}\]
\[\textbf{г)}\ \frac{1}{2} + \frac{1}{4} = \frac{3}{4} \cdot 1\frac{1}{3} = \frac{3}{4} \cdot \frac{4}{3} =\]
\[= 1 - \frac{5}{6} = \frac{1}{6}\]