You can add a network switch to connect more network devices.

>I currently use a cable company modem with a router built-in. This router has 4 ports where I connected computers in my office. So it allows me to have all computers (connected to the router) to be on the same network. So I can share drivers, printers, etc. I may need more that 4 ports (if I buy a network printer, for example). So I would need to buy another router with, say, 4 ports. Then by connecting the second router to the first one (the cable-modem router) I will increase number of devices I can connect to the network. Does it make sense?