Q vs pressure

if i have water line 2 inch cpvc pressure gauge reading 4 bar how to know the GPM approximate

please can you explain more or give some information because all the system is 4 bar pressure

4 bar is only the static pressure. The flow rate depends on the layout of the entire piping system between where the pressure is applied and the exit. Things like the total length of pipe, the internal roughness of the pipe (which changes over time) and the number of bends and how they are laid out in the pipe all effect the overall flow rate.