What is the old flow meter? Most of them other than the High Flows are not very accurate. 4.5l/min is 270 l/hr which is pretty high if you are pushing coolant through multiple blocks.. Without knowing anything about your loop, it's hard to say but that seems awfully high. What were your pump speeds set to when your old flow meter reported 4.5 l/min?

As for the 0 readings, the minimum the High Flow Next can report is 0.7 l/min (42 l/hr). Is it possible that at 50% pump speed your flow rate is under this? I have 2 High Flow Next's in a test loop. The loop consists of a D5 Next, one 360mm x 30mm rad, a spinner flow indicator, a ball valve, and the 2 High Flow Next flow meters. Tubing is 10mm/16mm (soft). At 50% pump power I get about 225 l/hr (3.75 l/min).The 2 High Flows report within 1.5% of each other which is better than the spec (+/- 2.5%). They get less accurate at low flow rates. When I add a CPU and GPU block I expect the flow rate to decrease significantly.