GoKawiilIf your router connects to a separate modem or optical network terminal (ONT), the Ethernet cable from that device normally plugs into the router's WAN or Internet port. WAN stands for wide area network; here, it is the router's connection to your internet service. For router/modem combo units, you won't need this connection since one device performs both functions.
The other Ethernet connections are generally LAN (local area network) ports. These are where you connect devices inside your home, such as a desktop PC, game console, smart TV or network-attached storage (NAS). Four LAN ports don't give the router four separate internet connections; they let you connect up to four wired devices directly (rather than using wireless connections).
Not every router uses that exact arrangement. Some have ports marked WAN/LAN that can perform either job, while other models use auto-sensing Ethernet ports instead of a dedicated WAN jack. If your router doesn't clearly identify one port for an Internet or WAN connection, its setup guide or specifications should tell you which port to use.
The number of LAN ports you need depends on how many devices you want to wire directly to the router. If you run out, an Ethernet switch can add more connections without requiring another router. Much like an HDMI switch, these plug into one of the LAN ports on your router and provide additional ports for other devices. You might need more than you'd think, given there are ways to use wired Ethernet without running long cables.