3 and 4 can't coexist.
to trial this (applies to a non-BootCamp VM only):
a) back up your VM
b) make sure you have your Fusion 3 install disk (or image) around
c) uninstall Fusion 3 (but make sure you keep your VM files)
d) install Fusion 4 - make sure you use the "upgrade from 3" icon on the trial disk image
e) start the VM
If you don't like Fusion 4, there are several threads here talking about how to revert, and there's a kb article, too. Example:
What is the proper procedure to downgrade from Fusion 4 to 3.1.3?
I'm not sure how to do this with a BootCamp VM.
