The definition of a "router" and "switch" can get confusing in the residential space with all the options from Linksys, DLink, netgear, etc. These devices are actually BOTH.
When you buy a cable/dsl router from Best Buy (for instance), with one of the brands above, they're called "routers." However, the 4 built in LAN ports are actually switched interfaces (its a built in 4-port switch). You've also got a WAN/Internet port (or 2) to connect your ISP to.
What you want is a switch, or if you want an all-in-one device form Linksys/Netgear/DLink/etc that you'll connect your ISP modem to on the WAN port and connect all your internal computers & printers to, then you'd need to find one that has 5 or more LAN ports and connect it all up. Any traffic that flows from computer-to-computer or computer-to-printer on those LAN ports will be "switched" traffic. If your computers communicate with the internet, this is considered "routed" traffic where the routing capability of the device is invoked.
If you already have an internet "router" connected and you just need more ports to connect all these devices together on your LAN, you can grab a cheap 8 port non-managed netgear 1Gbps switch for like $50, connect one of its ports to a LAN port on your router and connect all your internal devices to the remaining switch ports (and anymore open LAN ports on your router; they'll all communicate internally still).