Don't buy new copies >.<
just RAF yourself twice instead, and get the triple xp +mounts etc.

You don't need to install more copies of Wow on your machine, you can run 4 from the one install you have.
In some cases it has been beneficial to duplicate the wow folder 4 times, and run each toon from their wow folder to keep their settings etc separate.

There are a number of models to follow, and most are documented in the wiki (link in the toolbar above, between chat and gallery)