Depends on the physical geometry of your setup, but if you only have a small number of systems to connect, there are electronics store guys who can sell you a small router that would take four or eight Ethernet cables. As long as you can set up the router at a place in the office where the cable-run to each computer FROM THE ROUTER is no more than 100 meters (328 ft), you should be good. The shorter, the better. If you have a home network from your Internet Service Provider, you probably already have a router so you wouldn't be afraid of what they look like.
For a purely local connection, a "cat-5e" cable (it would have that designation on the blister pack) could easily handle 10 Mbit networks, 100 Mbit, and 1000 Mbit. The speed of the router will govern the fastest network you can handle, but the odds are very high that 100 MBit Ethernet would be adequate for a four-member network, and that isn't a very expensive router these days. "Cat-6" cables are newer and have better specifications including a higher network speed, but may be a case of overkill for a small office. Both cable types are common as dirt to purchase from any decent electronics store or even at your local WalMart, which might also have a decent small router that can go above 4 sockets for the network cables.
There MIGHT be a temptation to hook up your in-house network using WiFi. Resist that temptation. WiFi is first not as secure as hard cabling (i.e. your signal is being broadcast and WiFi security encryption isn't as robust as other types of hardware), and second isn't as resistant to radio static as a good, hard bit of copper cable. You absolutely DO NOT want a "bouncy" WiFi network as a business backbone because WiFi networks are associated with database corruption when a WiFi connection bounces for just a moment. We could discuss it for a long time, but the short answer is that WiFi is as dangerous as cloud drivers but for different reasons. The safest and most efficient connection is the hard-wired Ethernet connection.
There IS such a thing as a fiber network, but for what you described, that is another case of overkill and it is also less likely to be available on smaller office-oriented systems. You would need a special router and special interfaces on each computer for a fiber network. Internally they would be zippy-quick. For the U.S. Navy's Enterprise Data Center, we had Gigabit Ethernet and 6-8 Gigabit Fiber Channel for our network-attached storage devices, but that was to run a facility that housed over 1000 servers and a few Petabytes of disk space. So if you see some reference to fiber networks, they exist and run great - but you need a Congressionally approved budget to run them.
Other factors to consider would be whether you wanted folks to have Internet access from those workstations, because that adds a connection to the router to support that external access. Be warned that if you DO desire such Internet access, you GUARANTEE that you will need a good antivirus package for each computer. Otherwise, you will need to get a router with firewall and security screening abilities (which starts to get really expensive) and you start getting into the issue of having an IT person whose job it would be to keep everything properly configured and safe. In essence, an in-house-only network is pretty safe. The exact moment that you introduce the outside world to your environment, your security problems skyrocket. Note that if you intented to connect a thumb drive to any computer, a good antivirus package is also seriously necessary because the thumb drive probably has been connected to the outside world.
You asked about putting the network together. You will have to go through the process of adding each computer to the network one at a time to enable Windows File and Printer sharing. That should be enough for the in-house network. It isn't quite plug-n-play... more like plug-n-pray - but it isn't hard to add computers to networks. There is a networking wizard that will guide you through that. Always set them up to allow what is called DHCP - dynamic host control protocol - because that makes it CLOSER (but still not identical to) plug-n-play. The router is pretty much guaranteed to understand DHCP. Note that some routers allow you to use your computer to connect to the router's internal control setup; i.e. they have a networked dialog mode rather that making you buy an extra terminal for the router itself. There is also an option to allow each computer to auto-detect what speed its connection can handle, and it will automatically select the optimum speed, based on a quick handshake between your computer's network card and the router's network hardware, so again if you pick the correct options, that's another plug-n-play type of feature. Some printers have Ethernet connection capability which would allow you to put that printer on the network and share it, too. Its setup would also involve DHCP, most likely.
When you consider adding external networks, the network setup will get exceedingly complex quickly because suddenly you have to deal with routing tables and that is decidedly not automatic. Remember, the more you want to do, the quicker the price escalates, as well as the complexity of setup. You may, if you wish, show this reply to the boss who asked the question, and relay any questions. I DO, however, prefer to avoid private messaging. It is the goal of this forum to spread answers - which spread better when not privately offered. So ask away if you wish. Since I'm retired, I'm here fairly often - but I also have a life away from the forum, so I'm not here round-the-clock.