Sorry for the confusion the message is correct, the gateway is NOT the ip address on your eth4 interface.
The gateway needs to be the address the ISP has given you as the default gateway for the service where the eth4 address is connected.
We are sending the packet out eth4 with a destination gateway of that ISP address.
Converting from ScreenOS to Junos is a translation process with some manual effort. Juniper has a top level tool to help generate configuration from the existing file here.
https://i2j.juniper.net/s2j/index.jsp
From here you need to manual make adjustements as needed for the lines not found and also do some clean up and manual interface changes as well.