I hope this can help.
Sometime Fortigate is used as branch's default gateway, and at that branch we are using multiple VLAN and multiple switches.
Please see attachment for topology
It is quite challenging since fortigate GUI is very limited. Not all feature shown using GUI.
Below is the example of how to solve.
FG080D3915001432 # config system switch-interface
! -- enter system switch-interface
FG080D3915001432 (switch-interface) # edit switch3
new entry 'switch3' added
! -- create new switch interface, named 'switch3'
FG080D3915001432 (switch3) # set vdom root
FG080D3915001432 (switch3) # set member port3
FG080D3915001432 (switch3) # set member port4
! -- set switch interface member.
FG080D3915001432 (switch3) # next
FG080D3915001432 (switch-interface) # end
FG080D3915001432 # config system interface
FG080D3915001432 (interface) # edit switch3
FG080D3915001432 (switch3) # set vlanforward enable
FG080D3915001432 (switch3) # set l2forward enable
FG080D3915001432 (switch3) # next
! -- make the switch3 to forward vlan and l2 traffic
FG080D3915001432 (interface) # edit vlan4
new entry 'vlan4' added
! -- create VLAN 4 interface
FG080D3915001432 (vlan4) # set interface switch3
! -- map VLAN 4 to use switch3 interface (port3 and port4)
FG080D3915001432 (vlan4) # set vlanid 4
! -- assign VLAN tagging ID
FG080D3915001432 (vlan4) # set ip 192.168.30.1/24
! -- assign IP Address
FG080D3915001432 (vlan4) # set allowaccess ping
! -- set allow ping
FG080D3915001432 (vlan4) # set vdom root
FG080D3915001432 (vlan4) # next
FG080D3915001432 (interface) # edit vlan5
new entry 'vlan5' added
! -- create VLAN 5 interface
FG080D3915001432 (vlan5) # set interface switch3
! -- map VLAN 5 to use switch3 interface (port3 and port4)
FG080D3915001432 (vlan5) # set vlanid 5
! -- assign VLAN tagging ID
FG080D3915001432 (vlan5) # set ip 192.168.40.1/24
! -- assign IP Address
FG080D3915001432 (vlan5) # set allowaccess ping
! -- set allow ping
FG080D3915001432 (vlan5) # set vdom root
FG080D3915001432 (vlan5) # next
FG080D3915001432 (interface) # end
thats all. Good luck
