Let me see if I can explain this, while all other answers given are valid as well.
so you have 4 NIC's, and therefore you have 4 subnets configured, as the mask of the subnets will determine the size of the network the server can send packets to (directly).
If a situation occurs that the server wants to send a packet to an address which is not directly attached to the configured subnets, it needs a "gateway" to send it to.
Every NIC properties has a field called "Default Gateway" this is a little bit misleading, and comes from a time where a server would usually only have a single NIC.
One or more of the subnets which are configured may have a router (which is a gateway)
So you need to determine what router to use if a packet need to be send to an address outside of the configured subnets.
If you only have one router, on one subnet, it would be simple: on the NIC connecting to that subnet you put in the IP address of the router in the "Default Gateway" box. And you leave this field blank for all the other NIC's
Once you have done that, all packets which are destined outside the 4 subnets will be sent to that router, as this router is now the "Default Gateway" for the host.
It will get a bit more complex if you have more than one router, on more than one subnet. If this is your case, then you need to add multiple gateways to the routing table using the
ROUTE ADD command. And you do need to put a
metric in to determine which one is the "Default Gateway" (which is the one with the lowest metric).
So there is only one "Default Gateway" for a given host, it is the "Gateway" with the "Lowest Metric" in the "route table".
Easy config: only fill in the gateway in one of the NIC's and leave the other NIC's blank.
Advanced config: add routers as gateways specifying
metric with the
ROUTE ADD command.
you can use the
ROUTE PRINT command to check the route table to see how this is configured.