You can have both a 4.3 client and a 3.3.5 client if you want. Just download the torrent from this post http://www.twinkinfo.com/topic/24674-335-hamcakeland-genuine-level-19-twink-server/ - then you've finished downloading it, extract the files with WinRAR and place in it a different folder than your retail folder. Remember to name the new folder something like WoW 3.3.5.