I am just installing a water-water heat pump, open system. The pump is 19KW, I need about 4 m3/h of water from the producer request. What pump to use from the heat pump to the tank ( i use 400l tank)? 2 m3/h is ok?

You are looking for a pump that produces around 9 gpm (2 m3/hr) at a few feet (say 1 m) of head (pressure)?