Water Supply pressure
It's a trick question, you can never get 4 bar of pressure with only a 20 m head.

The pressure due to a vertical column of liquid is equal to the density x height of the column x the acceleration due to gravity (little g = 9.8 m/s2).

Your height is 20m

Density of water is 1000 kg/m3

g = 9.8

So, pressure is 196,000 Pascals 

100,000 Pascal per bar, so you have 1.96 bar as the max pressure you could get, independent of flow rate or pipe size.