\[\boxed{\mathbf{4.}}\]
\[\textbf{а)}\ 7 \cdot 2 = 14\]
\[7 \cdot 4 = 28\]
\[7 \cdot 3 = 21\]
\[7 \cdot 6 = 42\]
\[\textbf{б)}\ 14\ :2 \cdot 5 = 7 \cdot 5 = 35\]
\[16\ :4 \cdot 7 = 4 \cdot 7 = 28\]
\[30\ :6 \cdot 7 = 5 \cdot 7 = 35\]
\[18\ :3 \cdot 7 = 6 \cdot 7 = 42\]