5 has a lot of cool features that 4 didn't...just little things.
1. some nicer soft synths included
2. loads down the CPU noticably less
3. I could go on and on...there used to be a page on the cakewalk website that outlined all the minute differences...they really do add up to a big difference though...I guess it all depends how in depth you are with the program. The more I use it, the more things I find that are badass.