To express the total cost \( y \) in terms of the number of kites purchased \( x \), we can break it down as follows:
- The cost of each kite is $4, which means for \( x \) kites, the cost will be \( 4x \).
- There is a flat shipping rate of $16, which is constant regardless of the number of kites purchased.
Putting it all together, the equation for the total cost \( y \) in terms of the number of kites \( x \) is:
\[ y = 4x + 16 \]