Simple
1: you need a 802.1q aware switch;
2: cisco
config t
int gi 1/0/1
switch mode trunk
switch trunk allow vlan 1,20,30,40
end
3: FGT
config system interface
edit intf10
set vdom root
set ip 10.10.10.1 255.255.255.0
set interface " port1"
set vlanid 10
next
edit vlan20
set vdom root
set ip 10.10.20.1 255.255.255.0
set interface " port1"
set vlanid 20
next
edit networkblah
set vdom root
set ip 10.10.30.1 255.255.255.0
set interface " port1"
set vlanid 30
end
The name of the sub interface could be label in whatever fashion that you desire ( vlan##, name, dept,etc....)
oh, btw the fwpolicies,dhcp,vpn-interface,etc....will be conduct against the names that you set for the 802.1q tagged interfaces.
Also multiple ISP on one tagged interface would not be a great ideal if your investing into multiple ISPs for redundant connections.