BackTrack 2 is outdated, so unless you have a specific reason to run BackTrack 2, don't.
BackTrack 4 is the latest, although beta. If you feel it is stable enough for your needs or you want to help out fixing bugs, use this!
BackTrack 3 is stable.
Just keep in mind that BackTrack 4 changed - it will now be based on Debian contrary the older versions which were based on Slax -> Slackware.