The surface of water in a water tank located on the top of a house...

Height of the water surface above the tap, $h = 4 \text{ m}$

Density of water, $\rho = 1000 \text{ kg m}^{-3}$

Acceleration due to gravity, $g = 10 \text{ m s}^{-2}$

The gauge pressure of water at the tap is given by the hydrostatic pressure formula:

$P = \rho g h$

$P = 1000 \times 10 \times 4 = 40000 \text{ N m}^{-2}$

It is necessary to state that the tap is closed because the hydrostatic formula $P = \rho g h$ is valid only for fluids at rest. If the tap is open, water will flow and the pressure at the tap will decrease due to the increase in kinetic energy, in accordance with Bernoulli's principle.