You need to drop the existing default route:
/sbin/route del default gw 192.168.1.4
By hand: Set up a route to the gateway machine at your ISP end, then make this the default gateway:
/sbin/route add -net gateway netmask 255.255.255.0 dev eth1
/sbin/route add default gw gateway
Or you could turn on DHCP on eth1 (this is the easier option, as it also takes care of things like DNS for you).