\[\boxed{\mathbf{4}\mathbf{.}}\]
\[3 + 3 + 3 + 3 + 3 + 3 = 18\]
\[3 \cdot 6 = 18\]
\[5 + 5 + 5 + 5 = 20\]
\[5 \cdot 4 = 20\]
\[4 + 4 + 4 + 4 + 2 = 16 + 2 = 18\]
\[6 + 6 + 6 = 18\]
\[6 \cdot 3 = 18\]
\[2 + 2 + 3 + 3 + 3 = 4 + 9 = 13\]
\[5 + 4 + 5 + 4 = 10 + 8 = 18\]
\[3 + 3 = 6\]
\[3 \cdot 2 = 6\]
\[5 + 5 = 10\]
\[5 \cdot 2 = 10\]
\[6 + 5 = 11\]