plz help with dual internet link setup :(

story is:
office; 4 departments.
some problems with our ISP
we want to transfer one of the departments on other ISPs internet link.

network: cisco1841 , firewall Fortnet/fortigate
Windows server 2003:DC, AD, DNS, DHCP

scheme is:
global IP <=cisco=> 192.168.4.0 (unfirewalled network;PBX) <=fortigate=> 192.168.3.0 (internal firewalled network: DC, workstations)

so how it’s possible to transfer only one department out of 4 to other ISP link, BUT(!) leave it in one domain AD??????

cisco has only 2 ports, in my case wan/lan
Firewall has 2 WAN ports.

So how it’s possible to leave all the deps in one domain, use same old network resources?
How to switch networks, in dep, if one of the links are down, easily ?