Something like:
set service nat rule 10 destination address 98.x.x.x.
set service nat rule 10 destination port 80
set service nat rule 10 inbound-interface eth1
set service nat rule 10 inside-address address 192.168.2.1
set service nat rule 10 log disable
set service nat rule 10 protocol tcp
set service nat rule 10 type destination
As you don't have a dNAT rule on WAN (eth0), you're using port forward tab. First try on that tab to add both eth1 and eth3 as LAN interfaces