Concept:
For a vertical rectangular surface submerged in a liquid,
the hydrostatic pressure distribution is triangular.
The centre of pressure is located at
\[
\boxed{
h_{cp}=\frac{2h}{3}
}
\]
below the free liquid surface.
where
\[
h=\text{Depth of liquid}.
\]
Step 1: Write the given data.
Depth of liquid,
\[
h=4\text{ m}.
\]
Step 2: Calculate the centre of pressure.
\[
h_{cp}
=
\frac{2h}{3}
=
\frac{2\times4}{3}
=
\frac{8}{3}\text{ m}.
\]
Thus, the resultant hydrostatic force acts
\[
\boxed{\frac{8}{3}\text{ m}}
\]
below the liquid surface.
Therefore, the correct option is
\[
\boxed{(C)\;\dfrac{8}{3}\text{ m}.}
\]