Network expansion questions

Greetings, network gurus.

A friend of mine who owns a retail store has asked me to help him expand
his in-store network. His situation is as follows:

The store has 3 POS computers, one of them is the main server.
Additionally, there is a computer in the office. So altogether 4
computers, they're all connected to a Linksys 4-port router which in turn
is connected to the internet via DSL. Because the POS computers need to
access the main server, the POS vendor has disabled DHCP in the network,
and assigned IP addresses to each POS manually. I have followed suit with
the office computer, which so far has worked flawlessly.

Now the store is to install 2 additional Globalpayments terminals which
would be connected to internet in order to verify payments. Since all 4
ports on the Linksys router is fully occupied, there's a clear need to
expand the network. My questions are as follows:
1. Should I buy an 8-port router to replace the current 4-port one, or
alternatively, buy an 8-port switch and connect it to the existing 4-port
router?
2. The second option is considerably cheaper than the first, as I saw a
8-port Cisco 10/100 switch advertised locally for under $50. But I'm not
sure if the router+8-port switch combo approach is inferior to the 8-port
router solution in terms of network quality. Additionally, I wonder if
it's any potential complications to manually set device IP address on a
network with a router and a switch, as opposed to a network with just a
router.
3. Lastly, do I need some special cable to connect a router to a switch?
Is there any special setup required to tell the router that a switch is
attached to it?

Thank you for your expertise in advance.