Hello,
To enable QoS/SQM only on one of the LAN ports, you will need to
use port-based VLANs.
Configure port 4 to be on a separate VLAN in Network -> VLAN -> Port-Based, and create an interface for it in Network -> Interfaces -> General.
Ensure that this interface is on a different network than your main LAN. So if your main LAN is on the 192.168.1.0/24 network, your VLAN needs to be on a different one.
In the physical settings of the interface, attach the VLAN by choosing the
appropriate interface. For example, if the VLAN has an ID of 10, the interface will be eth0.10.
You can find more information about VLAN configuration here.
Once you have configured the VLAN, the new interface should become available in the drop-down menu on the QoS/SQM configuration page.
Additionally, if you want to manage communication between your VLAN and other firewall zones, I recommend taking a look at our wiki page here.
Kind Regards,
Andzej