|You'll need to research what software or OS you will be using to do what you want|
Servers are not cheap either. Since you are using 4 PC's, a simple home router can block the sites and limit activity times.
www.opendns.com is a great site to use for web filtering. Just point your router to their DNS servers
However, limiting download speed requires some sort of packet shaper.
We have a building with ~100 computers (not counting wireless clients), 3 web servers, an Untangle server and 6 internal servers.
Our untangle allows us to packet shape for certain computers (ie allows me to set speed for certain computers), web filtering, virus protection, spam protection, firewall, web cache, etc. But realize, this solution is expensive. Our server running this costs ~$6,000 plus subscription fee for untangle.
Since you only have 4 computers, I would not worry about packet shaping/throttling and use one of the first 2 solutions for web filtering. My opinion is to use OpenDNS