multiple WAN, Single GW, Traffic

[pille]

hello all,

i need following:

4 x Public IP (lets call them 1 2 3 and 4 with the gateway 9
4 Internal networks (lets call tham a b c and d)

in DNS are different IPs for each service (like web on 4, email on 2, ..) the internal DNS is on c
the traffic should go in 1 in, and only to the internal network a and send the answer to a out (not over 3 where the standadd Gateway is configured at the moment)
1 <> a
2 <> b
3 <> c
4 <> d
how can i do that ?
with centos its easy. just create custom routes which are attached to the interfaces. but how can i do it with opnsense. i am right now choosing the new firewall i want to use because of that requirement. opnsense looks very robust and has good references / so, i would love to use that software.
anybody an input ?
thx
regards