Looks like there may be an issue with the server; Torrents work though.
Try going to the downloads page and clicking the BackTrack 4 Final Release Torrent button. Using software such as Transmission (Mac OS X and Linux) or utorrent (Windoze) will get you the distro .iso file.
Oh yeah, on the downloads page, you will also find the md5 hash of the .iso. Ensure that once you've finished downloading, you check the md5 sum of the .iso you downloaded against the one listed on that page.