What does it mean if an internet domain name starts with something like "www4" instead of just "www"?

it has something to do with proxy. i'm not really sure.
i just know kids use it to get around site blocking on school computers.

"Many web addresses begin with www, because of the long-standing practice of naming Internet hosts (servers) according to the services they provide. So, the host name for a web server is often www as it is ftp for an FTP server, and news or nntp for a USENET news server etc. These host names then appear as DNS subdomain names, as in "www.example.com". The use of such subdomain names is not required by any technical or policy standard; indeed, the first ever web server was called "nxoc01.cern.ch",[ and many web sites exist without a www subdomain prefix, or with some other prefix such as "www2", "secure" etc. These subdomain prefixes have no consequence; they are simply chosen names. Many web servers are set up such that both the domain by itself (e.g., example.com) and the www subdomain (e.g., www.example.com) refer to the same site, others require one form or the other, or they may map two different web sites."
Source and further information:
http://en.wikipedia.org/wiki/Www#WWW_prefix

High traffic websites will have several different computers serving their websites. When you access www.company.com, you will be invisibly redirected to one of a number of different machines, and these server machines may well be called www1.company.com, www2, www3, etc. If you see a URL like that, you have caught sight of a bit of the internal machinery of the website working. You should generally not use such addresses, because they may change or become inaccessible without warning.

It's just a random domain name ordered by website manager, they can be whatever, don't even have to start with WWW.