Let's try to sort this. I assume your default policies are all set to ACCEPT. Use "iptables -L" to check this. Your flushing shoul be OK. If not you will see rules in the output of "iptables -L"
Then try this
iptables -t nat -A PREROUTING -i eth1 -j DNAT --to-destination 10.2.4.2
This shoul have immediate effect. No service to restart.
What this does is just change the destination (to 10.2.4.2) of the packets which reach the interface eth1 (18.104.22.168)