When you run the installation for Fusion 4, you are given 2 choices for the upgrade.
The top on is to drag the VMware Fusion icon and drop it on your Application Folder.
The other is to "Double-click to upgrade from VMware Fusion 3".
Which one did the people who had no activation problems use?
I've double clicked to upgrade the Fusion 3, no need to re-activate Windows at all.
Fusion 4 instruction specifically says to use the upgrade option in case there is previous version of Fusion installed on the computer.
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I have a 2010 13" MBP with 8gb of ram. It had a faster value prior to the upgrade. I'll have to play with the settings when I get home.
I would also start with the settings. Good luck.
Fusion 4 running already for two days and no problems at all. As I've said, it looks to be faster and smoother.