I never heard of Backtrack 4 until I saw your post here. I did a quick google, and from what I read I believe Backtrack is not a program you can install, but rather it's an actual live CD or live DVD that you can download and iso image of.
The ditsro itself has moved from Slax to Debian. Install at your own risk, but it definitely isn't suited to being used as a desktop distro. Run it from a live CD or USB key and go with Backtrack 3. Seems to be more stable and it's more well supported.