How do you connect a USB modem to a router?

This is beyond my limits of knowledge. I use computers but have no real interest on how they work. All this technical guff about connecting this and connecting that, just seem

Where can one purchase a Gigabit Ethernet cable?

 There are several retailers that sell Gigabit ethernet cables, including Office Max, Office Depot, Newegg, and Amazon. These products can also be purchased at select discount

Can you connect a router into a network that uses a switch?

The answer is yes. If you need more information, please be more specific on what it is you're trying to do.

Yes. Generally you plug your internet connection into the router, then an ethernet wire from the router to the switch. More than likely, depending on what set-up you previously had, your going to have to change your tcp/ip settings. I could probably guess what set-up you have... but you'll have to be more specific before we can give more advice.

Can you connect four switches to one Router?

Yes. Just connect one switch strait into the router. Then hook the other three switches into the first switch.