You want to use the piaware on the Pi4 then?
So you need the IP address of the Pi running the 978 stuff, let’s say 192.168.2.55
Then you use these commands on the Pi4
sudo piaware-config uat-receiver-type other
sudo piaware-config uat-receiver-host 192.168.2.55
sudo piaware-config uat-receiver-port 30978
sudo reboot
This should do the trick.