The short answer is that the ports labeled 1-4 are part of an Ethernet switch and are intended to be connected to PC's or other Ethernet devices to make a LAN.
The longer answer is that some routers can be flashed with new firmware that allows you to use each port as you wish. My preferred firmware is dd-wrt but there are others. The dd-wrt firmware allows you to designate more than one port for WAN links, or split the LAN links into two or more VLAN's, for example.