If I were you I would double check the physical connections. Maybe you can try to move the cable from zone 4 to one of the zones that are working (i.e. zone 3) to discard any problems with netro in port 4.
if the problem persist, that means that Port 4 is not the problem, then I suggest to use a multimeter to verify again the connectivity between netro and the valve.
Have you already verified that the problem is not the lack of water, right?