Remember that
The area of a rectangle is given by the formula
[tex]A=L*W[/tex]In this problem
we have
L=4,000 m-----> convert to kilometers --------> L=4,000/1,000=4 km
W=5,000 m ---------> W=5,000/1,000=5 km
Find out the area
[tex]\begin{gathered} A=(4)*(5) \\ A=20\text{ km}^2 \end{gathered}[/tex]