It's not really, you can just embed four streams anywhere and have a main streamer capture that, it just requires some good communication and a good internet connection from all of the parties involved.
A tip would be to set up your own site rather than using that third party "multitwitch" thing. That way you can set it up to look just like stream client, or in any other way you'd like it. If you need help with that I can both set it up and host it for you (although this should work on localhost as well), but I wont be streaming anything myself as I'm frankly not all too interested.
i.e (disregard the horribly distasteful color scheme, this is nothing but an example)
Live examples: 250x250 streams http://boaty.org/stream250/ 350x350 streams http://boaty.org/stream350/
A tip would be to set up your own site rather than using that third party "multitwitch" thing. That way you can set it up to look just like stream client, or in any other way you'd like it. If you need help with that I can both set it up and host it for you (although this should work on localhost as well), but I wont be streaming anything myself as I'm frankly not all too interested.
i.e (disregard the horribly distasteful color scheme, this is nothing but an example)
How to embed twitch streams
<object type "application/x-shockwave-flash" height="250" width="250" id="live_embed_player_flash" data="http://www.twitch.tv/widgets/live_embed_player.swf?channel=channel name here" bgcolor="#000000">
<param name="allowFullscreen" value="true" /><param name="allowScriptAccess" value="always" /><param name="allowNetworking" value="all" /><param name="movie" value="http://www.twitch.tv/widgets/live_embed_player.swf" /><param name"flashvars" value="hostname=www.twitch.tv&channel=channel name here&auto_play=true&start_volume=25" />
</object>
<param name="allowFullscreen" value="true" /><param name="allowScriptAccess" value="always" /><param name="allowNetworking" value="all" /><param name="movie" value="http://www.twitch.tv/widgets/live_embed_player.swf" /><param name"flashvars" value="hostname=www.twitch.tv&channel=channel name here&auto_play=true&start_volume=25" />
</object>
Live examples: 250x250 streams http://boaty.org/stream250/ 350x350 streams http://boaty.org/stream350/