Unless the water is flowing into a pressurized container, it will have to discharge at atmospheric pressure. This means that between the water main (@ 4bar gage) and the outlet (atm), the liquid will have to lose 4 bar of pressure. It can lose the pressure in two ways. Losses due to static pressure loss from elevation change (outlet is higher in elevation than inlet) and dynamic pressure losses due to friction (from the water flowing through the pipe, through valves, filters, etc).
You can easily determine the static loss expected by looking at the elevation difference between inlet and outlet.
The friction loss calculation is a bit more involved, but basically: a pipe of a given material will experience a certain amount of non-reversible energy loss (realized by reduced pressure) for a given flow rate. This is true of fittings, valves, filters, elbows, etc. It's a balancing act. More flow means more pressure loss per foot of pipe. Less flow means less losses. Your flow rate is determined by the configuration of your system and how much water needs to flow through your pipes in order dissipate enough energy that the liquid leaves the system at atmospheric pressure.