\[\boxed{\mathbf{4}\mathbf{.}}\]
\[\textbf{а)}\ 27 - 2 + 10 = 25 + 10 = 35\]
\[36 + 3 - 10 = 39 - 10 = 29\]
\[50 - 1 - 3 = 49 - 3 = 46\]
\[25 + 5 - 1 = 30 - 1 = 29\]
\[42 - 2 - 5 = 40 - 5 = 35\]
\[58 + 2 - 20 = 60 - 20 = 40\]
\[\textbf{б)}\ 57 - 20 + 13 = 37 + 13 = 50\]
\[6 + 34 - 27 = 40 - 27 = 13\]
\[100 - (5 + 9) = 100 - 14 = 86\]