Hello!
Usually all 4 Ethernet/LAN ports can be used to connect devices to the Internet but sometimes ISPs reserve one or two for services like IPTV, STBs, security systems, etc ... ... these are usually marked as well though not in every case ... one can try to see if it works by just plugging in the devices since it should mostly be working as soon as its plugged in ... if it does not, then, one can login in to the router's interface and check to see if there is some port binding or if there's another reason like not using a DHCP server (very unlikely for consumer hardware)! If more ports are needed, one can always use a switch as mentioned above!
Regards,
Ahmed