BackTrack 5 (R1) is the only version officially supported at the moment. I'm sure you can find a torrent for BackTrack 4, but you might as well just use BackTrack 5.
Once you have the ISO file installed on your PC, download a free program like Express Burn, insert your disc, and add the ISO file to a new "Data CD" you will be prompted to name the disc, fix your options, and choose a write speed. Keep your write speed low (around 4K) as higher speeds tend to cause problems with the data.
After the disc is burned, you will be able to use it like any other. Just make sure your MD5 code matches. You can find the correct code on the link below, and you can get your individual code using a program like HashCalc. Make sure they match, if they don't, redownload. Make sure you check the MD5 before burning the ISO.
Good luck! And no, this is not illegal in any way.