\[\boxed{\text{4.\ 370}}\]
\[\textbf{а)}\ 9,9 + 4\frac{5}{7} - 3,9 - 2\frac{3}{7} =\]
\[= 6 + 2\frac{2}{7} = 8\frac{2}{7}\]
\[\textbf{б)}\ 2\frac{1}{6} - 5\frac{5}{11} - 10,5 + 7\frac{1}{3} =\]
\[= 2\frac{1}{6} + 7\frac{2}{6} - 10\frac{1}{2} - 5\frac{5}{11} =\]
\[= 9\frac{3}{6} - 10\frac{1}{2} - 5\frac{5}{11} =\]
\[= 9\frac{1}{2} - 10\frac{1}{2} - 5\frac{5}{11} =\]
\[= - 1 - 5\frac{5}{11} = - 6\frac{5}{11}\]
\[= 7\frac{12}{18} - 4\frac{3}{18} - 3\frac{12}{24} =\]
\[= 3\frac{9}{18} - 3\frac{1}{2} = 3\frac{1}{2} - 3\frac{1}{2} = 0\]
\[= 5,8 - \left( 1\frac{20}{15} - 1\frac{7}{15} \right) =\]
\[= 5\frac{4}{5} - \frac{13}{15} = 5\frac{12}{15} - \frac{13}{15} =\]
\[= 4\frac{27}{15} - \frac{13}{15} = 4\frac{14}{15}\ \]