Fusion 4 DOES NOT WORK with Macintosh computers with the "Core Duo" chip. These are the 1st generation MacBook/ MacBook Pro computers after Apple switched processors from the PowerPC/ IBM chips to the Core Duo/Intel chips in 2006.
So, if you have a Mac that was manufactured from January 2006 to September 2007, Fusion 4 will not install on it, nor upgrade from any earlier version, which is my case.
Make no mistake, if you have a "Core Duo 2" chipset and up, according to the tech support supervisor I spoke to, you are able to install Lion (10.6) and Snow Leopard.
Additionally, as of yesterday, (11-18-11), you're able to install Snow Leopard as a virtual OS alongside Lion. But, VM reminds the user to respect the Apple EULA before installing.
I was going to hold off reviewing Fusion 4 until I'm able to upgrade my computer system, but it was too important not to tell 1st generation MacBook Pro owners of this requirement.
I bought Fusion 1 several years ago and upgraded every year. This is one of the most reliable of softwares out there.