The referenced howto should explain everything necessary. By default you should have individual interfaces for wan, lan1, lan2, lan3, lan4 with the later four lan interfaces bridged (in software). The easiest approach would to remove one of them from the land bridge (br-lan) and configure it individually as your second wan port.
The more advanced approach would be to follow the DSA Mini-Tutorial to set up bridge-vlan filtering and split the ports into multiple VLANs (offloaded to the switch hardware).