I asked this question a month or so back and was told that basically they are the same. Most of the differences are in the "student review" section at the end. The main content is the same. I just stuck with 2003 for what it's worth.
I know someone who took his first aid 2003 to the barnes and noble down the street and sat there all day, copying (by pen) new 2004 stuff into his 2003. 😀