FortiGate Next Generation Firewall utilizes purpose-built security processors and threat intelligence security services from FortiGuard labs to deliver top-rated protection and high performance, including encrypted traffic.
This article explains that due to hardware limitations on certain FortiGate models only physical interfaces are available for configuration. Due to a web-based manager limitation on the FortiGate 40C, VLAN configuration can only be configured on the CLI. 

FortiOS v4.0 MR3 and above.

Only physical interfaces are available for configuration on the following FortiGate devices:-
FortiWiFi 20C
FortiGate 30B
FortiWiFi 30B
VLAN configuration instructions for the FortiGate 40C
In the System>Network>Interface menu of the Web-based manager, the <create new> option for VLAN configuration is not present as shown below :
This is because to configure a VLAN interface this must be done on the CLI.
1. To create a VLAN interface on the FortiGate 40C connect via the CLI and configure the following :-
#FGT40C-3 # config system interface
#FGT40C-3 (interface) # edit test
#new entry 'test' added
#FGT40C-3 (test) # set interface internal
#FGT40C-3 (test) # set vlanid 100
#FGT40C-3 (test) # set ip
#FGT40C-3 (test) # set vdom root
#FGT40C-3 (test) # end
2. To verify whether the VLAN interface has been created, reconnect to the web-based manager in the System>Network>Interface menu and check the configuration.