\[\boxed{\mathbf{4}\mathbf{.}}\]
\[18 + 13 = 18 + 10 + 3 =\]
\[= 38 + 3 = 41\]
\[16 + 15 = 16 + 10 + 5 =\]
\[= 26 + 5 = 31\]
\[29 + 12 = 29 + 10 + 2 =\]
\[= 39 + 2 = 41\]
\[37 + 14 = 37 + 10 + 4 =\]
\[= 47 + 4 = 51\]
\[28 + 24 = 28 + 20 + 4 =\]
\[= 48 + 4 = 52\]
\[35 + 26 = 35 + 20 + 6 =\]
\[= 55 + 6 = 61\]
\[49 + 15 = 49 + 10 + 5 =\]
\[= 59 + 5 = 64\]
\[48 + 23 = 48 + 20 + 3 =\]
\[= 68 + 3 = 71\]
\[57 + 25 = 57 + 20 + 5 =\]
\[= 77 + 5 = 82\]