Answer
$4$ mL
Work Step by Step
If the saline has $ 4\%$ concentration, it means that $ 96\%$ is water, $4\% $salt...
Set up a table:
$\left[\begin{array}{ccc}
\% \text{water} & \text{mL of } & \text{mL of water}\\
& \text{ the solution} & \\
100\% & x & x\\
94\% & 8 & 7.52\\
& & \\
96\% & (x+8) & 0.96(x+8)
\end{array}\right]$
The equation to solve is:
$ x+7.52=0.96(x+8)\qquad$ ... multiply with $100$
$100x+752=96(x+8)$
$ 100x+752=96x+768 \qquad$ ... add $(-96x-752)$
$4x=16$
$x=4$ mL
