Pipeline Calculations

I have an unknown capacity of water supply at a specific supply waterline. I know the point where I want to tie-in is 4bar. I want to tie-in with a 4" line and then calculate the pressure and flow at different points in my new pipeline. I want to use the Bernoulli equation, but I am not sure how to determine the volume flow into the 4" pipeline from the 4bar supply line. I need some point of reference in the beginning. I tried various methods and then I keep getting different results. Is there a formula that combines the flow rate, pipe diameter and pressure? I have been using Q=VA, but then I don't know what the influence of the 4bar line would be on that connection point.

Regards