Switching a Physical Interface and a VLAN on a SoftSwitch

Hi everybody, this is my first post. I'm trying to connect a network that is on a phisical port and a vlan that is configured on a SoftSwitch. How can this be made?    i have the following config Fortigate 100E port 16 --> conected to a network SoftSwitch on ports 1-4, with VLAN3   What I need is that devices connected to ports 1-4 can see the network on port16, but i dont want my VLAN1 be accesible from port 16   Thanks in advance