We use 1 for management/legacy iscsi network, the other 3 are aggregated into our "ISCSI network".
Justin, can you clarify on the Layer 3 routing and network topology? If the second network that is not physically presented to the ESX server is not routable, then it seems logical this is due to the discovery of the other network when the ISCSI initiator on the ESX server is enumerating the NetApp filer. To confirm this, SSH into the ESX server and run the following:
/usr/sbin/vmkiscsi-ls
Post the output minus and "sensitive" data.