To my knowledge, the soundtrack for Saw IV recycle most of the music from I,II and III.
Saw III only had one maybe two new songs, which was the Shithole theme and maybe another, and saw IV had only one originally composed song.
So its best to download the original saw soundtrack, by Charlie clouser and nitpick what songs are on the other soundtracks.
Thread 46951
scroll to Charlie Clouser (C section) there are Saw I and Saw III for download, courtesy of haloofthesun.
Andrew =]