I just installed new CS 5.5 software. Following the instructions, I did not first de-install the previous CS 4 version. I assumed that the installation of CS 5.5 would automatically de-install the prior version, but it did not. I found no advice concerning the need for deinstalling the prior version. Why? Doesn't it take up system resources? Or should it remain on the system in order to support documents created with the prior version? I have a Macbookpro with OS X, ver. 10.6.8. BTW, my partner is a PC IT guy, who thinks one should keep a tidy ship!
It depends on which applications you use and how much compatability you need with older versions. There is no need to remove CS4 other than it might free up a small amount of disk space.
There is also a good chance that the file associations for different Adobe documents will be affected after the removal of CS4.