IP v6 could save you some bandwidth, but unfortunately this protocol is not widely supported yet. So for 4 users you will need 4 times more bandwidth than for a single one.

You may buy a hosting that will be able to handle your bandwidth, install a free icecast streaming server and enjoy it.

You may also broadcast with free p2p stream broadcasting software PeerCast. Of course peer2peer technology saves lots of bandwidth of a broadcaster using it.

There are also proxy caching solutions, but they must be too expensive in your case.