\[\boxed{\text{4.\ 214}}\]
\[\textbf{а)} - 20 = ( - 15) + ( - 5)\]
\[\textbf{б)} - 20 = - \frac{1}{5} + \left( - 19\frac{4}{5} \right)\]
\[\textbf{в)} - 20 = - 6\frac{3}{8} + \left( - 13\frac{5}{8} \right)\]
\[\textbf{г)} - 20 = - 7,9 + ( - 12,1)\]