LAN Designed based around ADSL Link

a switch is just a smart hub (it detects the MAC address of the attached device, and routes the packets that way, thereby avoiding broadcasting to every node)
a switch should be faster (provided they are both the same speed 10mbit or 100mbit)

Your Broadband router will hold the public IP address.  There is no need for ICS.  Plug whatever 3 workstations you want into the switch that is built into the router and take the last connection and plug it into the hub with the other computers.  Your Broadband router I'm sure by default will have DHCP enabled.  Simple and easy.  The Router does NAT, hands out IP addresses to all the workstations and everyone can access the Internet.

Cybor, BTY, a hub is a layer 1 device and works at the physical layer.  Whatever comes in will go out all ports.  It is 1 Broadcast and 1 Collision domain.  A switch is a layer 2 or layer 3 device operating at the Data or Network layer.  It is intelligent in that it knows the MAC address of what is connected to it's ports, thus traffic for something for port 2 only goes through port 2, leaving the other ports unaffected.  So, for a 4 port, layer 2 switch, it would be a 1 Broadcast and 4 collision domain device.  They are much better options now than hubs.

Cybor,
Since you are changing your network, are you also going to buy an antivirus product for the other 2 pcs, assuming that the PC with internet access already has it?

I would like to suggest another route to go which is not that expensive, allows for network growth and greater security.

I would just install a proxyserver on the computer that is going to have the usb connection to the internet. You wont need to buy a switch to share the internet this way.

I can personally recommend Winproxy. The cost for 3 users would be about $60 us.

It has a firewall built in, does all virus scanning BEFORE anything hits your network (using either Panda or McAfee you decide) You can do site-filtering, spam-filtering and content filtering. Set time and protocal restrictions per group or machine. Best of all your can download the full version and test it free for 30 days. You get free phone support for those 30 days.

Here is the link: http://www.winproxy.com
Check it out and see what you think. If you dont like it, then you can go the ICS route. The price alone is equivilent to the 2 VS products you would need for the 2 additional users.

I have used it here at work for over 2 years, and it has been great!

PS. also if you should decide that PC #4 needs email only access you can configure that easily.