Find the sum, use the summation capabilities of a graphing utility to verify your result.
To find the sum $\sum _{k=1}^{4}c$
$\begin{array}{rcl}\sum _{k=1}^{4}c& =& c+c+c+c\\ \sum _{k=1}^{4}c& =& 4c\end{array}$
Using graphing utility
Area of rectangle is
$\begin{array}{rcl}A& =& 4\times c\\ A& =& 4c\end{array}$