Because we're treating the swimming pool as a standard rectangle, rather than a three dimensional shape, we can use the formula L * W = A to calculate the length of the pool.
Length * Width = Area
We've established two key bits of information, and with them we can solve for the final piece.
Length * 4 = 16
4l = 16
Divide both sides by 4.
l = 4
The length of the pool is 4 meters.