For OSX it's not necessary. For OS 9 it also 'might' not be necessary. As far as I can see there's just a limewire package in the Library Receipts folder but I'm not sure whether that's related to OS 9 or X.

In windows it can cause problems b/c one version may inadvertantly run in the background. But for mac OS 9 I think you should be safe. However, you might have problems if you jump backwards from version 4 to 3 with your downld preferences. I can't swear that would happen but might. Your downld preferences are stored in a file in your incomplete folder called download.dat & dwnld.bak. So if you intend to jump back to version 3, make a backup of these files 1st.