got quite a similar layout (but other routing):
eth0 takes 192.168.1.0/24 (my private) and eth1 takes a internal lan (192.168.2.0/24) with a connection to the www.
i tell my kernel/ethers how to route:
(ip) route add -net 192.168."1".0 netmask 255.255."255".0 eth0 ..
route add -net 192.168."2".0 netmask 255.255."255".0 eth1 ..
route add default gw 192.168.2.1 (the gate to www somewhere in the lan).
when i now say to a client (somewhere in 192.168.1/2.x) that the router is the default gateway, the client sends his packet there. the router decides where to put the packet and sends it out over the right interface. he knows, that he can reach 192.168.1.x on eth0 and 192.168.2.x on eth1.
i think you have to give the ethers other IP-ranges (like above .1.0&.2.0). netmask should be 255.255.255.0 on both eth1 and eth3.
hope i could help a bit. bye ritch.